This paper took the constant-breadth cam mechanism with floating flat faced pushrod as research object. The concepts of rise and return travel, radius-making line, and width mapping interval set were introduced. The intrinsic regularity relationship of the angles of rise/return and farthest/nearest dwell angle, and the characteristic relationship of kinematic sizes, amplitude of rise travel, and the beginning parameters were discussed in detail. Then, the displacement function of the rocker was solved. The selected interval of the flat and the range of base circle radius, pressure angle, and width were presented. They reveal that the design of this mechanism contained a special, complex and profound content. The comparison of constant-breadth cam and conjugate cam were presented, which deepen the understanding of similarities and differences of the two kinds of shape lock mechanism. A mechanism synthesis example of this mechanism was displaced.