The invention relates to a gear ring adjustable internal combustion engine variable compression ratio mechanism, and belongs to the technical field of engines. The mechanism is characterized in that a positioning table is fixedly connected to the left side of the upper part of a gear ring; a belt gear wheel I of a crankshaft group is connected with a belt gear wheel II of a phase adjustment group through a belt; a gear I and a gear II of the crankshaft group are meshed with the internal tooth of the gear ring; a gear III of a revolving speed synchronization group is meshed with the external tooth of the gear ring; a helical gear I of the revolving speed synchronization group is meshed with a helical gear II of the phase adjustment group. According to the mechanism, continuous compression ratio variation of the internal combustion engine can be realized; besides, the structure is compact, the installation space is small, and moreover, with the utilization of the timing belt space and the balance shaft space of the internal combustion engine for arrangement, the installation space is reasonably utilized.