The invention discloses an isothermal relaxation current principle-based basin-type insulator nondestructive testing device and method. The device comprises a shielding box, wherein the outer wall of the shielding box is grounded, the shielding box is provided with a cover capable of being opened and closed, the shielding box is provided with an opening hole for mounting a cable adapter substitute, the cable adapter substitute is connected to a fixed contact of a multiway selection controllable switch, and a moving contact of the multiway selection controllable switch is communicated with a high voltage polarization direct current power supply and then is grounded, or is communicated with a ground wire, or is communicated with a discharge current limiting resistor and then is grounded, or is communicated with a micro current measurement module and then is grounded; a signal output end of the micro current measurement module is connected with an upper computer, and the upper computer is also connected with the high voltage polarization direct current power supply and a control terminal of the multiway selection controllable switch. A system has the advantages of high testing sensitivity, simple and convenient operation, good anti-interference performance, nondestructive testing, no injury to a human body and low cost, and the testing sensitivity for defects causes by cracklike area type defects, linear metal inclusion, and poor bonding at an interface between a metal insert and casting resin is higher.