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By discussing the structure, working process and the key parts of 2BSF-4 irrigating-sowing machine, this paper analyzed the principle of irrigating-sowing mechanism. The sowing mechanism has a signal generator, and the irrigating mechanism has a signal actuator. The signal that emerged in seeding process is transmitted to the irrigating mechanism, and controls its action. The sowing mechanism and the irrigating mechanism can work in same rhythm and phase with this technique. The sowing mechanism, the irrigating mechanism and the valve, can make seed and irrigating water drop into the dynamic soil cavity which behind the plough. This technique can make seeds and water in same level. In working process, this machine can test seeds and water situation avoiding leakage sowing and leakage irrigating.