A mechanical rotary steering drilling tool for steering a drill string includes a test section, a clutch device, a control mechanism, and an execution part. The test section is used as an upper joint in cooperation with the tool. The clutch device is externally connected with a guide body and internally coupled with a mandrel. A groove on a cylindrical surface of a switch control cylinder is matched with a control screw to determine rotation of a fluid channel switch. A pushing block of the execution part extends out against a wellbore wall to push a drill bit for steering drilling after the fluid channel is opened.