The C4 fraction hydrogenating process includes the following steps: mixing C4 fraction material and hydrogen and contacting the mixture with hydrogenating catalyst I in a hydrogenation reactor for diolefin hydrogenating saturation reaction, contacting the reaction resultant with hydrogenating catalyst II directly without separation for impurity eliminating and olefin hydrogenating saturation reaction, and cooling and separating the reaction effluent to obtain the C4 fraction hydrogenating product. The process has mild reaction condition and can eliminate most of olefin from C4 fraction material, and the C4 fraction hydrogenating product with olefin content lower than 1.0 vol% may be used as high quality ethylene producing material and vehicular liquefied gas.