For the development of heavy oil reservoirs，the recovery principle and method of steam injection in dual-horizontal well for the heavy oil reservoirs proposed by BUTLER in 1991 has been paid more attention by the experts and scholars at home and abroad. However，the definition of SAGD is inappropriate，because the gravity drainage contributes less than the injection pressure during the process of SAGD. Therefore，the definition of GASD（Gravity-Assisted Steam Drive） was proposed. In the process of BUTLER’s derivation of the productivity formula，steady flow of the Darcy’s law has been mistaken as differential flow and several wrong solutions existed during the formula derivation，which results in the inaccuracy of SAGD productivity calculation formula. Based on the model of movable oil formed by steam chamber proposed by BULTER，the tilted linear flow formula of Darcy’s law was used to obtain the GASD productivity calculation formula for dual-horizontal wells. At the same time，important results were achieved and show that the GASD productivity is in direct proportion to the sinθ and is in inverse proportion to the vertical distance between the dual horizontal wells.