Since inquiry was first introduced to science education, a lot of inquiry-based tasks have appeared in textbooks for supporting an inquiry-based approach. These tasks have great potential to direct the implementation of an inquiry-based approach, as teachers rely heavily on the use of existing instructional materials in most K-12 classrooms. However, these tasks would not actually take effect unless they could accomplish the mission of assisting in the achievement of the educational goals of an inquiry-based approach, for which it is necessary to assess the design quality of inquiry-based tasks in textbooks. Content analysis is an effective way to evaluate the design quality of textbooks. This study adopted the instrument -- the Inquiry-based Tasks Analysis Inventory (ITAI), which fits the purpose and has strong reliability and validity -- to analyse the inquiry-based tasks in current high school biology textbooks in Mainland China. The results show that (1) some of the inquiry-based tasks separate the inquiry process from scientific content, (2) current textbooks do not present balanced dispositions to use inquiry process skills, and (3) inquiry-based tasks in current textbooks do not reflect a proper understanding about scientific inquiry. The findings call for revisions of inquiry-based tasks in textbooks and indicate the ambiguous understanding about inquiry that may be held by the textbook authors. What is more, the problems revealed by this study are very similar to those found in previous studies in other regions, which highlights the need for continuous attention to the evaluation of inquiry-based tasks in textbooks.