你不贴代码没看出问题
eqns = {S2''[z] - a2^2 S2[z] + (a2^2 - a1^2) Sa == \[Gamma]/e,
S1'[z] - a1^2 S1[z] == \[Gamma]/e};
sol = DSolve[eqns, {S1[z], S2[z]}, z]

eqns {Subscript[s, 2]''[z] -
Subscript[\[Alpha], 2]^2*
Subscript[s, 2][
z] + (Subscript[\[Alpha], 2]^2 - Subscript[\[Alpha], 1]^2)*
Subscript[s, a] == \[Gamma]/e,
Subscript[s, 1]'[z] -
Subscript[\[Alpha], 1]^2*Subscript[s, 1][z] == \[Gamma]/e};
sol = DSolve[eqns, {Subscript[s, 1][z], Subscript[s, 2][z]}, z]

又好了，不知道为啥

明白了，前边定义过函数了，后边好像又套回了

还有个问题就是，为啥我从文件-新建笔记本，重新输入代码那个IN［］不是从1开始，必须重新打开软件才是从1开始呢

https://tieba.baidu.com/p/6149871454