Abstract: This paper analyses some productivity forecast equations of horizontal well,specially Joshi’s equation,used in reservoir engineering,thinks that the effective permeability should be used in calculating horizontal well productivity,points out the error existing in Joshi’s equations,and then derives a new equation for predicting horizontal productivity applying mirror effect theory.Field applications show that the relative error between practical and calculated value is less than 17%,and that calculation result is more accurate than that of Joshi’s equations.