Compared with classroom learning, online learning requires students to self-regulate their learning processes and to maintain their motivation to achieve their learning goals. This study investigated whether the interventions based on the Attention, Relevance, Confidence, and Satisfaction model and the modified Introducing the new concepts, Metacognitive questioning, Practicing, Reviewing and reducing difficulties, Obtaining mastery, Verification, and Enrichment method can promote students' motivation and use of self-regulated learning (SRL) strategies in a web-based mathematics learning environment. Two-hundred thirty-six Chinese high school students were randomly divided into four groups: motivational design group, SRL intervention group, motivational design and SRL intervention group, and the control group. Questionnaires and tests were administered to measure the changes in motivation, use of SRL strategies, and academic gains before and after the respective interventions. Findings of this study suggest that the knowledge gain of students learning mathematics online can be maximized when they receive instructional assistance in both motivation and use of SRL strategies.