The invention discloses a simple preparation method of Relebactam intermediate, namely (2S,5R)-N-(1-protective group) piperidine-4-yl-6-benzyloxy-7-oxo-1,6-diazabicyclo[3.2.1]octane - 2- formamide, (2S,5R)-6-benzyloxy-7-oxo-1,6-diazabicyclo[3.2.1]octane-2-carbonyl chloride is prepared from (2S, 5R) -5- benzyloxaminopiperidine -2- formic acid and phosgene, solid phosgene or diphosgene in a solventin the presence of an alkali and a catalyst by epoxidation and acylating chlorination reaction by a one-pot method, and the (2S,5R)-6-benzyloxy-7-oxo-1,6-diazabicyclo[3.2.1]octane-2-carbonyl chlorideis not separated and is directly subjected to amidation reaction with (1-protective group)-4-amino piperidine to obtain the (2S,5R)-N-(1-protective group) piperidine-4-yl-6-benzyloxy-7-oxo-1,6-diazabicyclo[3.2.1]octane - 2- formamide. The method disclosed by the invention is simple in steps, cheap and easily available in raw materials, green and environment-friendly in process, low in cost, high in reaction atom economy, high in purity, yield and selectivity of the obtained intermediate, and beneficial to industrial production.