A kind of optimal path planning method for complete field coverage in agricultural machinery farming was described. The given convex field was decomposed into sub regions according to typical faming pattern of complete field coverage. Optimal path planning should minimize agricultural machines turns, farming time, route distance or other optimization object. According to the different purpose of path planning above, optimal travel direction could be determined based on topological features of arable lands, turns pattern in the headland and other useful information. Farming path planning software was developed by using this algorithm. The test results from four actual quadrangular fields showed that the optimization object for field operation in optimal travel direction decreased obviously compared with other directions. Each optimal travel direction in those quadrangular fields above was along any edge. Especially for a quadrangle having a larger edge ratio, the optimal travel direction was almost the longest edge. 