I recommend the following
#show heading: set par(leading: 1em)
#show heading: set block(below: 100000em) // Change as you wish
text.spacing controls the horizontal space between space-separated words. It makes no sense here.
Additionally, please use show-set rules in favor of show-function rules whenever possible.
Occasionally, you might want to override your previous rules. In that case, writing another show-set rule just works, while writing another show-function rule will get them nested.
More info: Which show rule takes precedence? - #15 by Y.D.X
Besides, since you write Chinese, maybe the following basic intro would help.