Disclosed is a process for preparing an intermediate of anti-tumor drug niraparib and an intermediate thereof. The present invention discloses a process for preparing compound f, which comprises conducting a cyclization reaction of compound e in a solvent and in the presence of a base to give compound f. The process of the present invention does not involve the steps of catalytic reduction or catalytic coupling reaction of precious metals and chiral separation, which has advantages such as low equipment requirements, simple operation, favorable industrial production, avoiding waste liquid containing heavy metals and phosphorus, low cost and high product ee value.