The invention relates to a valve mechanism for the scavenging air valve of an internal-combustion engine. The valve mechanism has at least a valve lever, wherein, the valve lever or each valve lever has a first end which is acted on at least one corresponding scavenging air valve and a second end which is hinged with a corresponding handspike, wherein, the handspike or each handspike is effectively linked with an adjusting device used for the phase shift of a valve train, the adjusting device has an adjustable rocker corresponding to each handspike, one side of the adjusting device is matched with an eccentric shaft, and the other side thereof is matched with a surface which runs on the cam of a cam shaft and is at least partially in a circular orbit structure. According to the invention, the adjustable rocker is connected with the handspike through an un-adjustable rocker, the un-adjustable rocker has an arc-shaped sliding orbit on the surface where the first end faces the adjustable rocker, and the arc-shaped sliding orbit is at least partially arranged into the structural form of being concentric with the base circle of the cam or being corresponding to the coaxial line, thereby the un-adjustable rocker can move along the base circle direction of the cam through the arc-shaped surface of the roller of the adjustable rocker.