By changing ratio of side length, the critical value of follower force of unstability of rectangular plate under uniformly distributed tangential follower force with different boundry conditions were  changed. The differential equation of motion of the rectangular plate was established. By using differential quadrature method, numerical results of the complex characteristic equation were calculated. Relations between frequency and follower force, influence of ratio of length on the instability form were induced. The results show that there are a secondary buckling behavior of square plate under follower force; keeping the side length perpendicular to the follower force, improving the side length parallel to the follower force, the critical value of follower force increases evidently. To the rectangular plate with SCSC boundry condition, when the ratio is under a certain value, instability of the plate will always be divergence, when it's beyond the value, instability of the plate will always be flutter.