This research applied a web-based dynamic assessment system, GPAM-WATA system, to help low English achievers to perform self-directed learning of junior high school English grammar. A quasi-experimental design was adopted. A total of 124 seventh graders from four classes participated in this research. Participants were randomly divided into the GPAM-WATA group and the paper-and-pencil test (PPT) group. A total of 33 students were identified as low English achievers, among which 17 belonged to the GPAM-WATA group and the remaining 16 belonged to the PPT group. All participants in the GPAM-WATA and PPT groups performed self-directed learning through the GPAM-WATA and the PPT, respectively. Only 33 low English achievers from both groups took the pre- and post-test of learning achievement assessment and the English learning motivation scale. The research findings reveal that low English achievers in the GPAM-WATA group exhibit significantly better improvement in terms of learning effectiveness than those in the PPT group. Moreover, it is found that the IPs in the GPAM-WATA are effective in compensating for low English achiever's lack of grammatical knowledge.