The invention provides a measurement device and measurement method for linearity of a flame face of a cylinder cover. The measurement device includes a bridge board and a gradienter. The bottom face of the bridge board fits with the flame face of the cylinder cover. The gradienter is arranged on the top face of the bridge board. The gradienter includes a housing, an adjustment lever, a laser emitter, a measurement cylinder and a slide rod. The bottom of the housing fits with the top face. The adjustment lever is in rotary connection with the inner side wall face of the housing. The adjusting lever includes a large end and a small end. The laser emitter is arranged on an end adjacent to the large end of the adjustment lever. The measurement cylinder is arranged in the housing. One end of the measurement cylinder is connected with the top part of the housing. The first end of the slide rod is in sliding fit with a fit segment on the other end of the measurement cylinder. The second end of the slide rod abuts against one end away from a rotary shaft of the adjustment lever. The fit segment is provided with scales in the axial direction. The middle position of the scales is zero point.Under the effect of gravity, the adjustment lever is always in a horizontal state. The first end of the adjustment lever is at the zero point. The housing is provided with viewing holes. The top partof the device is provided with an indication assembly.