Assignment[-((4 (-1 + r)^2 (-1 +
t) (t - \[Theta]) (-1 + \[Theta]) (t - 2 \[Theta] +
t \[Theta])^2)/(-M^2 (-1 + t)^2 (t (-3 + 4 r - \[Theta]) -
4 (-1 + r) \[Theta])^2 +
4 M (-1 + t) (t - \[Theta]) (8 (-1 + r) r \[Theta]^2 -
2 r t \[Theta] (-1 - 7 \[Theta] + r (2 + 6 \[Theta])) +
t^2 ((-1 + \[Theta])^2 + 4 r^2 \[Theta] (1 + \[Theta]) -
r (1 + 7 \[Theta]^2))) -
4 (t - \[Theta]) (-4 \[Theta]^2 (-4 + 4 r + 4 \[Theta] -
4 r \[Theta] + r^2 \[Theta]) +
4 t \[Theta] (-4 - 2 \[Theta] + 6 \[Theta]^2 +
r^2 \[Theta] (2 + \[Theta]) +
r (4 + \[Theta] - 5 \[Theta]^2)) +
t^3 (-3 - 2 \[Theta] + 5 \[Theta]^2 + r^2 (1 + \[Theta])^2 -
2 r (-1 + \[Theta]^2)) -
t^2 (-4 - 15 \[Theta] + 10 \[Theta]^2 + 9 \[Theta]^3 +
r^2 \[Theta] (5 + 6 \[Theta] + \[Theta]^2) +
r (4 + 10 \[Theta] - 8 \[Theta]^2 - 6 \[Theta]^3))))),
r = 0.08, \[Theta] = 0.3, M = 0, 8]

楼主以前应该不是用MMA的吧
MMA里面对式子中的变量赋值的操作是采用替换函数:Replace或者其简写形式/.
代码形式如下:
-((4 (-1 + r)^2 (-1 +
t) (t - \[Theta]) (-1 + \[Theta]) (t - 2 \[Theta] +
t \[Theta])^2)/(-M^2 (-1 + t)^2 (t (-3 + 4 r - \[Theta]) -
4 (-1 + r) \[Theta])^2 +
4 M (-1 + t) (t - \[Theta]) (8 (-1 + r) r \[Theta]^2 -
2 r t \[Theta] (-1 - 7 \[Theta] + r (2 + 6 \[Theta])) +
t^2 ((-1 + \[Theta])^2 + 4 r^2 \[Theta] (1 + \[Theta]) -
r (1 + 7 \[Theta]^2))) -
4 (t - \[Theta]) (-4 \[Theta]^2 (-4 + 4 r + 4 \[Theta] -
4 r \[Theta] + r^2 \[Theta]) +
4 t \[Theta] (-4 - 2 \[Theta] + 6 \[Theta]^2 +
r^2 \[Theta] (2 + \[Theta]) +
r (4 + \[Theta] - 5 \[Theta]^2)) +
t^3 (-3 - 2 \[Theta] + 5 \[Theta]^2 + r^2 (1 + \[Theta])^2 -
2 r (-1 + \[Theta]^2)) -
t^2 (-4 - 15 \[Theta] + 10 \[Theta]^2 + 9 \[Theta]^3 +
r^2 \[Theta] (5 + 6 \[Theta] + \[Theta]^2) +
r (4 + 10 \[Theta] - 8 \[Theta]^2 -
6 \[Theta]^3))))) /. {r -> 0.08, \[Theta] -> 0.3,
M -> 0.8}
另外,楼主最后赋值M变量的地方错把小数点打成了逗号

特别感谢你，谢谢~

ReplaceAll[(8 (-1 + r) (-1 + t) (t - \[Theta]) (-1 + \[Theta]) (t -
2 \[Theta] +
t \[Theta])^2 (-M^2 (-1 + 2 r) (-1 +
t)^2 t (-1 + \[Theta]) (2 (-1 - r + 2 r^2) \[Theta] +
t (1 + 2 r - 4 r^2 + \[Theta])) +
4 M (-1 +
t) (t - \[Theta]) (-2 t (1 + r^2 (7 - 3 \[Theta]) +
r (-5 + \[Theta]) +
2 r^3 (-1 + \[Theta]) - \[Theta]) \[Theta] +
4 (-1 + r) r \[Theta]^2 +
t^2 (1 - \[Theta]^2 + r^2 (4 + 6 \[Theta] - 6 \[Theta]^2) +
2 r^3 (-1 + \[Theta]^2) +
r (-3 - 4 \[Theta] + 3 \[Theta]^2))) +
4 (t - \[Theta]) (t (-1 + \[Theta]) (2 \[Theta]^2 +
t^2 (1 + \[Theta]) - t \[Theta] (3 + \[Theta])) +
2 r (-1 + \[Theta]) (4 \[Theta]^2 - t^3 (2 + 3 \[Theta]) -
t \[Theta] (4 + 9 \[Theta]) +
t^2 (1 + 9 \[Theta] + 4 \[Theta]^2)) +
r^2 (-4 (-2 + \[Theta]) \[Theta]^2 +
t^3 (-3 - 2 \[Theta] + \[Theta]^2) +
4 t \[Theta] (-2 - 3 \[Theta] + 2 \[Theta]^2) +
t^2 (2 + 13 \[Theta] - 3 \[Theta]^3)))))/(M^2 (-1 +
t)^2 (t (-3 + 4 r - \[Theta]) - 4 (-1 + r) \[Theta])^2 -
4 M (-1 + t) (t - \[Theta]) (8 (-1 + r) r \[Theta]^2 -
2 r t \[Theta] (-1 - 7 \[Theta] + r (2 + 6 \[Theta])) +
t^2 ((-1 + \[Theta])^2 + 4 r^2 \[Theta] (1 + \[Theta]) -
r (1 + 7 \[Theta]^2))) +
4 (t - \[Theta]) (-4 \[Theta]^2 (-4 + 4 r + 4 \[Theta] -
4 r \[Theta] + r^2 \[Theta]) +
4 t \[Theta] (-4 - 2 \[Theta] + 6 \[Theta]^2 +
r^2 \[Theta] (2 + \[Theta]) +
r (4 + \[Theta] - 5 \[Theta]^2)) +
t^3 (-3 - 2 \[Theta] + 5 \[Theta]^2 + r^2 (1 + \[Theta])^2 -
2 r (-1 + \[Theta]^2)) -
t^2 (-4 - 15 \[Theta] + 10 \[Theta]^2 + 9 \[Theta]^3 +
r^2 \[Theta] (5 + 6 \[Theta] + \[Theta]^2) +
r (4 + 10 \[Theta] - 8 \[Theta]^2 -
6 \[Theta]^3))))^2 /. {r -> 0.2, \[Theta] -> 0.5,
M = -0.8}]
但是这样，为什么不可以？谢谢你