This paper analyses the three-period Bertrand-Stackelberg market price competition and the strategy of product differentiation location, and makes a static contrast with Bertrand-Nash market equilibrium; then shows the influence of product differentiation degree in a firm＇ s profit change in equilibrium in duopoly market through a concrete example. Based on this, the paper identifies and relaxes the supposed conditions in the classic model of product differentiation location to discuss the varying trend and stability problems of profit and price in equilibrium in monopoly market where the advantage of marginal costs, the customers＇ strategic action and repeated dynamic game all exist. Relaxing the supposed conditions of traditional models is helpful to understand many industrial competition problems that have been studied separately for a long time