中国大陆和美国有treaty,只要withholding 15%。Capital gain on stock 不用withhol

If you were in the United States for less than 183 days during the tax year, capital gains (other than gains listed earlier) are tax-exempt unless they are effectively connected with a trade or business in the United States during your tax year.

所以除了扣15% on dividends, interest income，对于non resident alien, 不用报税。