The invention belongs to structure connection devices for servo mechanisms and engines and particularly relates to a double crank type structure for rotational motion and torque transmission between a servo mechanism and a liquid engine. The transmission connection device for the servo mechanism and the engine comprises an output shaft crank which is connected with the output shaft of the servo mechanism through screws and a pin, a shift pin presses into a hole on the end portion of the output shaft crank and connects an input shaft crank and the output shaft crank, and the input shaft crank is connected with the output shaft of the engine through a large screw. The transmission connection device for the servo mechanism and the engine has the advantages of being capable of being adapted to high temperature deformation of the pendulum shaft of the engine during working; being capable of being used for transmission and connection of the rotary output servo mechanism and the engine; achieving the efficient transmission of the servo mechanism and the engine, wherein the transmission efficiency is approximate to more than 90% and the efficiency has no relation with a pendulum angle; reducing transmission parts and gaps between the servo mechanism and the engine; being small in size, easy to manufacture and low in costs.