Online assessment is the key issue of studying online learning during the COVID-19 pandemic. Often, online assessments are understood as the mirror to face-to-face learning assessments. In fact, online assessment should start from the premise that all interactive learning strategies are assessment strategies, and as such should be considered for their value in measuring the experiences of students and instructors. This study used the phenomenological method of qualitative research through in-depth interviews with 14 college students and 6 instructors. Based on this, it explored the perception of online assessment in Macao higher learning institutions from the complementary perspectives of college students and instructors, and constructed a theoretical model. The main research findings are that after the assessment shifted from offline to online, some new features appeared, and corresponding changes in form and function occurred. Formally, online assessment is manifested as single subjectivity and flexible diversity, which leads to the assessment dilemma of lack of constructive alignment. At the same time, the functions of assessment have also been extended and even alienated. So the effect of online assessment is not satisfactory. The recommendations for the development of the technological environment and the changes of formative and summative assessment are also discussed in this paper. In terms of the technological environment, efforts need to be made to provide more authentic online learning experiences with synchronous feedback and flexible interaction. In order to help develop a holistic person with online learning, which is the ultimate goal of learning, corresponding criterion-referenced assessment and functions of assessment should be improved accordingly to promote student learning while considering students' recognition and facilitate the co-construction of knowledge between instructors and students. In addition, future research can focus on the development of the readiness of assessment technology, as well as the form and purpose of assessment. Overall, this study provides a more comprehensive understanding of online learning and assessment from the dual perspectives of instructors and students, which could help further studies in this field.