This paper focuses on the design and evaluation of a theory-based computer-assisted summary writing learning environment called Summary Writing-PAL (SW-PAL). The SW-PAL was developed based on four aspects: summarizing strategies, learning theories, prior knowledge, and cognitive load. A quasi-experiment that involved 58 undergraduates majoring in Computer Science was conducted to examine the effectiveness of SW-PAL in writing summaries. Two intact classes were selected with 28 and 30 students in control and experimental groups, respectively. The conventional teaching approach was employed in the control group, whereas the SW-PAL was introduced to the experimental group. Pretest and posttest were administrated to both groups. The findings indicated that SW-PAL improved students' summary writing performance. A significant variance was noted between intrinsic and extraneous cognitive load among students with varying levels of English proficiency in the experimental group, signifying that the SW-PAL is more suitable for students with lower language proficiency.