A constant tension mechanism of a training machine is provided, including: a base, configured to be connected to a body of the training machine; an elastic member, abutted against the base; and a tension mechanism, including a movable member movably connected with the base and abutted against the elastic member, and at least one tension pulley rotatably disposed on the movable member and configured to urge a belt connected to the body; wherein relative to the base a direction in which the elastic member forces the movable member and a direction in which the at least one tension pulley moves point toward the same direction.