The form of six circular arc and egg-shaped cross-section is complex. The hydraulic element is divided into conventional transcendental equation given by four intervals. The surface line calculation by use of common method exists not only cumulative error but also heavy workload of calculation and low efficiency. Based on optimum and fit theory, took the minimum of standard remaining difference as the objective function, the paper got a simplified universal formula instead of the original integral and non-integral functions within the engineering application parameters obtained by the fitting calculation segmented (maximum fitting error only 1.53%). The method not only realized the solution conversion of differential-by-sectional into direct integration ,but also further improved the accuracy of the projections because of the positive and negative fitting errors cancel each other. Using the approximate formula to calculate the water surface line of the six arc egg-shaped section tunnel makes the solved process more simple and the work efficiency higher, and has the practical significance of promotion.