Reading-to-write (RTW) tasks have been commonly employed in second language (L2) English academic writing pedagogy, and many studies have investigated the validity and reliability of RTW tasks in L2 English writing assessment. Meanwhile, few studies have examined the cognitive validity of RTW tasks, and the design and validation of such tasks in L2 Chinese academic writing assessment remain underexplored. This study develops a Chinese RTW task following a set of design criteria and procedures and evaluates its cognitive validity as an instrument of L2 Chinese academic writing assessment. The RTW task was administered to 15 undergraduate and 15 postgraduate L2 Chinese learners in an eye-tracking laboratory. Analyses of the task features and the eye-tracking and stimulated recall interview data suggested that the RTW task largely aligned with the characteristics of authentic tasks in real L2 Chinese academic writing contexts and elicited a representative range of cognitive processes in existing models of RTW cognitive processes. Many of these processes manifested in different ways between the two groups of participants at different L2 Chinese proficiency levels. Our findings have useful implications for understanding the cognitive validity of the RTW task in L2 Chinese writing assessment.