This paper explores two issues of classroom SLA from the cognitive perspective: effects and acquisition mechanism. Firstly, it reviews a number of studies focusing on different effects of classroom SLA, followed by an analysis of their possible reasons from the cognitive perspective. Secondly, it elaborates how input, output, interaction and form-focused instruction affect the cognitive process of SLA respectively, with example of foreign language instruction in China. Lastly, the paper points out the implications of SAL cognitive mechanism for foreign language instruction in China.