The invention relates to a starting method of a hydrocracking unit. A reaction zone of the hydrocracking unit is filled with an oxidation type hydrogenation catalyst. The starting method comprises steps of heating circulating gas to (150-225)+/-15 DEG C, introducing a sulfurizing agent, performing sulfuration at a constant temperature of 230+/-15 DEG C, heating continuously to raise the temperature to 370+/-20 DEG C and performing sulfurization at the constant temperature, introducing low-nitrogen starting oil when the temperature decreases to 150+/-15 DEG C after the catalyst sulfurization is finished, raising the temperature to 210+/-20 DEG C and introducing nitrogen-containing light distillate oil, introducing nitrogen-containing heavy distillate oil instead when the temperature is 290+/-15 DEG C or higher, introducing raw oil instead step by step when the temperature is 320+/-15 DEG C and shifting to normal production. Compared with the prior art, the starting method replaces anhydrous liquid ammonia by the nitrogen-containing distillate oil and the starting process is smooth and steady.