The invention relates to a crank arm assembly and a power transmission self-locking mechanism using the crank arm assembly. The power transmission self-locking mechanism comprises a guide post which can move up and down in a guiding mode. The guide post is provided with a transmission groove. The crank arm assembly is arranged inside the transmission groove and comprises a power input shaft, a connecting shaft, an installing shaft and a jacking head, wherein the connecting shaft is perpendicularly arranged on the power input shaft, and the jacking head is connected to the installing shaft through a spring and used for being in pushing fit with the upper groove wall of the transmission groove in the guide post in a sliding mode. When the crank arm assembly is located at an extending locking station, the upward force is applied to the upper groove wall of the transmission groove through the jacking head, the downward force is applied to the lower groove wall of the transmission groove through the tail end of the connecting shaft, and locking is conducted on the crank arm assembly in the extending position. When the crank arm assembly is located at a retracting locking station, the upward force is applied to the upper groove wall of the transmission groove through the jacking head, the downward force is applied to the lower groove wall of the transmission groove through the outer peripheral face of the connecting shaft, and locking is conducted on the crank arm assembly in the retracting position. According to the crank arm assembly and the power transmission self-locking mechanism using the crank arm assembly, an additional locking mechanism for locking a crank arm in the prior art is omitted, and the structure is simple.