A calculation method for the conjugate water depth in open trapezoidal channel is presented. First of all, an iterative formula for the conjugate depth is given on the basis of  mathematical transformation of the basic equation of conjugate depth and its flow characteristics. Subsequently, by solving unary quadratic equation and cubic equation, an approximate formula for the initial value of conjugate water depth is obtained, based on which an expression for the initial value iteration is established. A direct formula is thus obtained by using the iterative formula. An illustrative calculation indicates that this method is simple and accurate, and the neither table nor  figure is needed to determine the empirical coefficients.             