The utility model relates to the field of engines and discloses a balance capacity variable engine balance mechanism, which comprises a balancing weight and shaft seat assemblies arranged at two ends of the balancing weight, wherein each shaft seat assembly comprises a shaft seat body and a spring tensioning device; the shaft seat body is provided with a chute of which the width is greater than that of a mounting part of the balancing weight; mounting parts at the two ends of the balancing weight are mounted in chutes; the spring tensioning device comprises a spring seat and a spiral spring of which one end is fixed in the spring seat, and the spring seat is arranged on the shaft seat body; and the other end of the spiral spring is fixed to the balancing weight. The balancing weight slides along the chutes to change the distance between the centroid and the rotating shaft of the balancing weight and change the balance capacity, and the vibration and noise of an engine are reduced and the reliability and durability of the engine are improved further. Because the balance capacity is variable, the balance mechanism with the same configuration has a wider application range, and can be applied to different engines to achieve the effect of quickly matching and developing an engine platform.