A kind of AC spot welding dynamic resistance real time measure instrument and method in the welding technology field, the said instrument includes: the welding transformer, a couple of inverse parallel silicon control, Holtz current sensor, minitype Lowe's loop, net pressure synchronization loop, current zero cross pulse detection loop, the silicon control starting circuit, welding current signal conditioning circuit, current differential coefficient signal conditioning circuit, sound-light alarm circuit, clock and replacement circuit, periphery drive circuit, keyboard and display circuit, communication and interlocked interface circuit, DSP control system. The method is that: uses embedded artificial NN calculate the angle of welding dynamic power factor and the current flow, then calculate the silicon control whole flow current virtual value, calculate the dynamic resistance in the spot welding process by using the ratio of the dynamic power factor and the whole flow current virtual value. This invention can detect the angle of welding dynamic power factor in quarter cycle, the instrument is simple, the real-time ability is good, the measurement precision is high.