三维偏移成像的核心问题是三维波场外推算子。本文在最小二乘意义下用共轭梯度方法导出优化系数的三维傍轴近似方程。这虽然具有经典45°的形式，却能使近70°的界面反射波准确地偏移归位，同时我们构造出了该方程的隐式、恒稳、中心差分的差分方程。凡是具有经典45°形式的三维偏移方程都可用此差分方程求解。与已有的三维一步法偏移算子相比，该算法具有效率高、稳定性好、处理效果明显及编程简单的优点。 The key issue of 3D migration imaging is the extrapolation of 3D wavefields. In the sense of least square, this paper derives the three-dimensional paraxial approximation equation of optimization coefficient by conjugate gradient method. Although it has the classical 45 ° form, it can exactly shift the interface reflection waves of nearly 70 °. At the same time, we construct the implicit, steady and central difference equations of this equation. Any three-dimensional migration equation with the classical 45 ° form can be solved by this difference equation. Compared with the existing three-dimensional one-step migration operator, this algorithm has the advantages of high efficiency, good stability, obvious processing effect and simple programming.