The invention discloses a hydraulic control type compression ratio variable mechanism. The wall of a piston skirt part is provided with two piston pin through holes. A connecting rod small head is located in a piston inner cavity. A connecting rod small head hole serves as a hydraulic oil cavity hole at the same time. A piston pin is installed in the piston pin holes and the connecting rod small head hydraulic oil cavity hole. The connecting rod small head hydraulic oil cavity hole is divided into an upper oil cavity and a lower oil cavity by the piston pin. A first oil channel and a second oil channel are formed in a connecting rod body to serve as oil supply channels of the upper oil cavity and the lower oil cavity. Compared with the prior art, the hydraulic control type compression ratio variable mechanism can adjust oil inlet operation and oil outlet operation of the upper hydraulic oil cavity and the lower hydraulic oil cavity according to the use conditions of an engine, thus theup-down position of the piston pin is adjusted, and finally the purpose that the compression ratio of the engine is adjusted is achieved. The engine can use a large compression ratio for a small load, and thermal efficiency is improved. The engine can use a small compression ratio for a large load, and knock is avoided. Power output of the engine during a low speed can be improved, thermal efficiency of the engine during a low load can also be improved, and the purpose of oil saving of the engine is achieved.