why?

error:
let .macro num1,num2
lda num2
sta num1
.endm

.org $5500
let $40,#$65

right:
let .macro num1,num2
lda #num2
sta num1
.endm

.org $5500
let $40,$65

If the first one is right,that is very useful.

Looks like your assembler is interpreting the macro arguments numerically, instead of making a verbatim text substitution. So, the "#" makes a syntax error in the first case but is legal in the second. I'd have expected an assembler to use simple text substitution in a macro, but perhaps that's not how this one works.

Could you tell us which assembler you're using, please? And could you also show us the complete error message that you got from the assembler?

Oh,I am using "6502 Macroassembler & Simulator "
I downloaded it's source code and wanted to change it,but my C++ is very poor~~