The present disclosure provides an insulation withstand voltage detection method and detection device for a wiring harness, and an automobile wiring harness assembly line. The insulation withstand voltage detection method for the wiring harness includes: step S10: placing a to-be-detected wiring harness in an enclosed cavity, in which a conductive electrode plate is disposed in the enclosed cavity, and the to-be-detected wiring harness has one end insulated and enclosed, and the other end connected to a power supply; step S20: reducing air pressure in the enclosed cavity; step S30: providing a test voltage by the power supply; and step S40: detecting a voltage of the to-be-detected wiring harness, and/or detecting a leakage current of the to-be-detected wiring harness. Through the present disclosure, it alleviates the technical problem in the prior art that the dielectric insulation detection of a high-voltage wiring harness cannot realize the in-line detection of finished products.