A Stirling engine includes a piston seat, a displacer arranged in the piston seat, a piston seat, and a push rod. Meanwhile, a piston tube is arranged on the piston seat and an airtight piston is arranged in the piston tube. Moreover, one end of the push rod is connected to the displacer, while another end passes through the airtight piston and extends out of the piston seat. Additionally, a power mechanism includes the aforementioned Stirling engine, a crankshaft, an axle seat pivoted with the crankshaft, and two pairs of first and second swing arms that are respectively connected from the crankshaft to the top ends of the push rod and the airtight piston and are arranged symmetrically to each other.