Through mechanism analysis of differential model of particle swarm optimization, the effect of the maximum speed constant is analyzed and the results are shown that can guarantee the existence of solution, but decrease the global search capability. A new broaden differential model is proposed, which treats the velocity and position vectors equally and searches the space at the same time. And the stability condition is also discussed. Thus a new modified particle swarm optimization algorithm is given. The optimization computing of some examples is made to show that the new algorithm has better global search capacity and rapid convergence rate.