Online learning is increasingly popular as the pandemic spreads around the globe. This shift in learning preferences presents opportunities and difficulties for the assessment of learning. As a method of assessment used throughout the learning process, formative assessment can encourage students' interest in learning, enhance learning outcomes, support teachers' strategic planning and maximize teaching results. However, the effectiveness of online formative assessment has been questioned. This study uses a qualitative research methodology based on Grounded Theory to analyze the validity of formative assessment in online learning by examining the factors unique to online learning that affect it. After reviewing previous studies and interviewing 45 online learners, this paper three-level-coded the sources and analyzed them. Findings show that the validity is influenced by the lack of functional alienation caused by the effects of the subject's emotional game and adaptability, objective environment and technology and poor interaction. To help implement formative assessment in real classroom practices, two models were established based on the findings, namely the Formative Assessment Model (FAM) and Online Formative Assessment Validity Model (OFAV). This study asks about the influence factors and explores how much the validity of formative assessment shows in online practice, and provides new feasible ideas and suggestions for the future development of the field of educational assessment.