As an approximate straight-line mechanism, the ladder-shaped four-bar straight-line linkage (ladder-shaped linkage) was named for its special configuration, which connecting-rod paralleled to the frame. Due to the complexity of existing path synthesis theories of ladder-shaped linkages, a graphical synthesis method was presented. Firstly, the research improved the previous research based on analytical computations of the Ball-Burmester point (Ball point) theory, and proved that under certain conditions, the special configuration and the degeneration of circle point curve on the linkage were necessary and sufficient conditions for each other. Secondly, based on this proposition, the geometric feature characteristic-circle was discovered, leading to the possibility of graphical synthesis. A synthesis method was developed. Given the position of Ball point and the direction of linear motion, the position of each hinge and the dimension of each connecting rod can be determined by planimetric plotting. Moreover, two ladder-shaped linkages in different dimensions can be synthesized at once by the same initial condition. The plotting steps were given, and some special cases in the plotting process were discussed. Matlab programs were compiled to verify the synthesis method in three different initial conditions, proving that the mechanism obtained the ability of drawing approximate straight line. Finally, The parameter line-ratio was proposed to measure the length of liner motion time. The line-ratio of linkages under three different initial conditions was listed. By genetic algorithm, the linkage with maximum line-ratio was explored, given a ladder-shaped linkage with a line-ratio of 0.665, which can be applied in quadruped and hexapod robots.