•A new in-line mode Mach–Zehnder interferometer based on photonic crystal fiber coated was designed.•The graphene quantum dots and polyvinyl alcohol film were used as the sensitive materials.•The MZI was formed by splicing a section of PCF between two sections of single mode fiber (SMF) with two up-tapers.•The measurement result revealed that different film thickness had different sensitivity and spectrum movement direction.•Meanwhile, the sensor had the advantages of simple structure, small size, high sensitivity and low cost. In this paper, a new in-line mode Mach–Zehnder interferometer (MZI) based on photonic crystal fiber (PCF) coated with graphene quantum dots (GQDs) and polyvinyl alcohol (PVA) film for relative humidity (RH) sensing was designed. The MZI was formed by splicing a section of PCF between two sections of single mode fiber (SMF) with two up-tapers. The two up-tapers acted as mode splitter/combiner and were made by fusion tapering technique. The Mach–Zehnder interferometer (MZI) incorporated inter modal interference between core mode and cladding modes of the PCF. The refractive index (RI) and volume of GQDs-PVA film will decrease and expansion with the increase of RH, respectively. By measuring the wavelength shift of the interference pattern, RH measurement could be achieved. Before RH measurement, the external RI was tested experimentally to verify that the interference spectrum shifted to right with the increase of refractive index (RI), and the sensitivity was up to 343.7 nm/RIU. In addition, the RH sensing properties with the different thickness of GQDs-PVA film (2.48 μm, 3.72 μm, 4.45 μm, 5.96 μm, 7.42 μm, 8.17 μm) were also investigated experimentally, and the corresponding sensitivity were -0.0901 nm/%RH, −0.0797 nm/%RH, -0.0337 nm/%RH, 0.0586 nm/%RH, 0.0539 nm/%RH, 0.0313 nm/%RH, respectively. The measurement result revealed that different film thickness had different sensitivity and spectrum movement direction. Meanwhile, the sensor had the advantages of simple structure, small size, high sensitivity and low cost. What’s more, the sensor also presents good stability and reversibility of the sensor.