The invention belongs to general measurement technologies and relates to a resistance welding spot concaving depth measurer and a measurement method. The measurer is characterized by being composed ofa vernier scale (1), an angle square (2), a support (3) and a locking screw component (4) and is mainly used for measuring the concaving depth of a welding spot after resistance welding. Through themeasurer and the measurement method, welding spot depth can be measured after two planar parts or two parts in arbitrary curvature are in lap joint for resistance welding, welding defects such as welding spot non-fusion, sealing-off and internal spatter caused by excessively large part assembling clearance and inappropriate technological parameters in the resistance welding process can be preliminarily determined, and therefore the problem that resistance welding lacks a reliable nondestructive detection method is solved. Tests prove that by adopting the measurer and the measurement method tomeasure the welding spot depth of resistance spot welding of a certain type of products, compared with the requirements of welding spot concaving depth stipulations, the welding defects of the weldingspot can be preliminarily determined, and the accuracy can reach 90% or above.