The rapid cooling during laser welding can prompt the occurrence of solidification cracking, however, few study is found on systematic research of solidification cracking during laser welding. It is the key to accurately measure the critical strain and temperature of the crack in order to know solidification cracking susceptibility. By using U-type hot cracking test with in-situ observation method, the occurrence of solidification cracking is captured clearly by high-speed-high-magnification camera. With the in-situ observation method, the local critical strain of crack is measured by tracking the displacement of two reference points near the crack; the local temperature of crack is measured by inserting the thermocouple to the trailing edge of the weld pool. Local critical strains at different temperatures are obtained under different tensile loads in U-type hot cracking test. The high temperature ductility curve is achieved accurately.