The utility model relates to a fuel cell stack voltage monitoring and diagnosing device which comprises a 121 needle vehicle-used connector, a measurement wire bundle and a fuel cell stack voltage measurement unit, wherein one end of the measurement wire bundle is respectively connected with an even electrode plate of a fuel cell stack or a trimerous electrode plate, and the other end of the measurement wire bundle is connected with the fuel cell stack voltage measurement unit through the 121 needle vehicle-used connector; the fuel cell stack voltage measurement unit is used for detecting the voltage information of each measurement point transmitted by the 121 needle vehicle-used connector, comparing the average voltage of the fuel cell stack and the voltage information of each measurement point and determining whether the voltage of a cell at each measurement point is normal. The fuel cell stack voltage monitoring and diagnosing device can achieve the voltage measurement and the diagnosis of the automobile fuel cell and has the advantages of low realization cost, simple structure, safety, reliability and the like.