Using plasma spraying technology to prepare coatings on the surface of TC4 titanium alloy parts is a common method to enhance the properties of titanium alloy, but the mechanism of coating failure during deformation is not clear. Therefore, the mechanical properties of NiAl-coated TC4 plate were tested through tensile tests at room temperature, and the characteristics of the mechanical behavior at each stage were analyzed in depth. Then, the cracks in the coating were observed by metallography microscope, the cause of coating failure was clarified, and a uniaxial tensile coating failure model was established. The results show that the failure of coating is caused by cracking and debonding. The cracking is the crack that runs through the thickness of coating, and the debonding is the separation caused by the mechanical bonding failure between matrix and coating. Cracking and debonding are two stages of the coating failure process. Cracking occurs in the elastic stage during the early stage of stretching, which occurs at defects in the coating or on the surface of coating under the action of stress, while debonding mainly occurs in the plastic stage during the later stage of stretching, the bonding failure between coating and substrate leads to the coating to fall off. The periodic cracking of coating causes the stress-strain curve in the elastic stage to show a periodic step shape, while the debonding and cracking of the coating in the plastic stage lead to part of the coating remaining on matrix.