The proton exchange membrane (PEM) water electrolyzer has such advantages as simple system, low operating temperature and small-scale hydrogen production according to real time requirement, and the hydrogen production process is clean, meeting the environmental requirements. The PEM water electrolysis hydrogen production is the reverse reaction of fuel cell, but the water electrolysis requires high operating voltage, the resistance is likely to generate a lot of waste heat, and the nonuniform current density results in hot spots, the internal temperature rises, accelerating the decomposition of hydrogen molecules, the water electrolyzer is likely to age and fail. In addition, four important physical parameters (temperature, flow, voltage and current) in the running water electrolyzer can influence its performance and life, but the present bottleneck is external, theoretical, simulated or single measurement, the authentic information in the water electrolyzer cannot be obtained accurately and instantly. This study uses micro-electro-mechanical systems (MEMS) technology to develop a flexible integrated (temperature, flow, voltage and current) microsensor applicable to the high voltage and electrochemical environment in water electrolyzer, which is integrated with a 20 mu m thick polyimide (PI) film material. The real-time microscopic diagnosis and measurement in the PEM water electrolyzer can measure the internal local temperature, voltage, current and flow distribution uniformity instantly and accurately, so as to optimize the operating conditions and analysis. (C) 2019 Elsevier Ltd. All rights reserved.