The objective of the current study is to examine the impact of an in-school computer-assisted learning (CAL) intervention on the math achievement of rural students in Taiwan, including a marginalized subgroup of rural students called Xinzhumin, and the factors associated with this impact. In order to achieve this, we conducted a cluster randomized controlled trial involving 1,840 fourth- and fifth-grade students at 95 schools in four relatively poor counties and municipalities of Taiwan during the spring semester of 2019. While the Intention-To-Treat (ITT) analysis found that the CAL intervention had no significant impacts on student math achievement, the Local Average Treatment Effect (LATE) analysis revealed significant associations with the math performance of the most active 20% of students in the treatment group. LATE estimates suggest that using CAL for more than 20 minutes per week for ten weeks corresponds to higher math test scores, both in general (0.16 SD-0.22 SD), and for Xinzhumin students specifically (0.3 SD-0.34 SD). Teacher-level characteristics were associated with compliance rates.