For gas cap reservoir with strata dip,mechanical property of oil-gas contact movement is described by particle-tracing of Lagrange. With assumption of straight line movement,movement of oil-gas contact is obtained by basic differential equations of oil and gas seepage. Application results show that: With decrease of reservoir pressure,decline of gas phase potential is much greater than oil phase potential; Oil-gas contact intends to move towards oil well as oil rim is only developed; On the contrary,oil-gas contact intends to move towards gas well as gas cap is only developed; Rational control of oil recovery rate and gas recovery rate is beneficial to stabilize oil-gas contact.