Computer-mediated explicit Instruction (CMEI) is effective in promoting second language development, particularly in the area of speech acts. However, more information is needed about the role of learners' language proficiency and perceptions in the effectiveness of CMEI. This study aimed to investigate the effect of CMEI on EFL learners' knowledge of speech acts, specifically apologies, thanks, and requests, and to examine the moderating role of language proficiency and learners' perceptions of the Instruction. A mixed-method research design was used. The study involved 180 EFL learners who were recruited from Wenzhou Medical University. The participants were randomly assigned to one of three groups: (1) the computer-mediated explicit instruction (CMEI) group, which received explicit Instruction on speech acts through a computer program; (2) the non-CMEI group, which received explicit Instruction on speech acts through traditional classroom instruction; and (3) Control group, which did not receive any explicit instruction on speech acts. The participants' language proficiency was assessed using the Oxford Quick Placement Test (OQPT) prior to the intervention. The quantitative phase employed the pretest-posttest design, and data were collected using a speech act recognition and production test and an interview checklist to measure learners' perceptions of the Instruction. Results showed that CMEL was more effective than non-CMEI. Participants also had positive perceptions of CMEI. Findings have theoretical and practical implications for English language teachers and applied linguists.