%let b= %substr(&a.,2,6);

其实，你应该用一个简单点的方法：

%let a = 200701;

%let b = "&a.";

或者用直接的方法：

%let a = "200701";

%let b = 200701;

I suggest you use the codes below, which need not macro language.
If you use %substr, you have to define a macro, which is boring.

%let a="200701";
%let b=%sysfunc(substr(&a,2,6));
%put a=&a b=&b;