Abstract: 20MnNiMo is the low-alloy steel forging which first developed in China for the main equipment of the nuclear island. It will be used in the pressure-bearing boundary of the main equipment of the Hualong2.0, such as the steam generator, pressure stabilizer and exhaust row. There is no engineering manufacturing experience for this grade of forging in China. It is extremely necessary to conduct research on the welding performance of 20MnNiMo low alloy steel and master its key welding technologies. This paper mainly studies the cold crack sensitivity of 20MnNiMo low alloy steel. Specifically, the cold crack sensitivity of the electrode arc welded joint is evaluated through the welding crack test of the inclined Y-shaped groove. Through calculation, it can be known that the carbon equivalent Ceq of 20MnNiMo low alloy steel is 0.5532, and the cold crack sensitivity index Pcm is 0.2962. According to the Graville diagram, 20MnNiMo is located in the weldable zone close to the difficult-to-weld zone. Therefore, 20MnNiMo low alloy steel has a relatively large hardening tendency, and cold cracks are prone to occur in the heat-affected zone during welding. According to the test results, preheating the 20MnNiMo low alloy steel at a temperature above 110℃ can prevent the occurrence of cold cracks in the heat-affected zone of welding. When the preheating temperature is below 110℃, cold cracks of different sizes appear. Without preheating, the crack rate of the cross-section is 100%. The test results are in agreement with the theoretical calculation results.