With the proposal of IAEA Specific Safety Guide No.SSG-39 on hardware verification and validation (V&V), relevant parties involved in nuclear instrumentation and control systems have gradually initiated research on hardware V&V. In the V&V system, the safety integrity levels determine the set and execution intensity of V&V tasks, directly affecting the effectiveness of V&V work. The allocation of integrity levels has always been a difficult aspect of V&V work both domestically and internationally. To address the challenge of allocating integrity levels during the hardware V&V implementation in engineering projects, this paper analyzes domestic and international standards and specifications related to integrity levels, conducts in-depth research on the key characteristics that affect hardware integrity levels, and studies mathematical models applicable to each of key characteristic, and ultimately determines the contribution of each key characteristic to the hardware safety integrity levels. Through quantitative calculations, a set of allocation scheme for hardware integrity levels applicable to the V&V of nuclear instrumentation and control systems is summarized. This scheme is scientific and innovative. It has been practiced in the nuclear safety-level Distributed Control System (DCS) of a Hualong NO. 1 supply unit of the China National Nuclear Corporation (CNNC). The feasibility and effectiveness of the scheme have been verified, and it can effectively guide the criticality analysis tasks of the hardware V&V for nuclear instrumentation and control systems.