Optical fiber sensors are widespread used due to the advantages of electromagnetic interference resistance, excellent sensing performance and minor size. Sensitivity enhancement is always a focus of research. Optical Vernier effect as an effective method to improve sensitivity has attracted significant research interest. The optical Vernier effect is available with the combination of two interferometers, place in series or in a parallel configuration. To employ the optical Vernier effect, the free spectral ranges (FSRs) are designed to be slightly different, which could be achieved by choosing their optical path length (OPL). However, the optical lengths of the sensing cavity and reference cavity must control carefully to realize the Vernier effect. This makes it much more difficult to realize optical Vernier effect. In order to solve the problem, two Mach-Zender interferences (MZIs) with high birefringence in polarization-maintaining fiber (PMF) is proposed to generate optical Vernier effect. This approach allows for the simultaneous adjustment of the optical lengths of two orthogonal MZIs by simply altering the length of the PMF in a single-arm configuration. By cascading the sensing cavity with the reference cavity within this single-arm structure, environmental interference is mitigated, which enhances the accuracy and reliability of the measurement process.This sensor is constructed by sandwiching a piece of PMF between two four-mode fibers (FMFs) (Fig.1). The FMF is used to excite the high-order modes. Because of self-focusing effect, the outgoing beam will appear two states of convergence and divergence. Through the analysis of simulation, the length of FMF is selected as 1.27 mm. The light from the beam divergence point of the FMF enters the PMF transmitting both in the core and clad (Fig.3). The PMF is used to obtain orthogonal MZIs. The interference generates between the high-order mode and two polarization modes ($ {\text{LP}}_{\text{01}}^{{x}} $ mode and $ {\text{LP}}_{\text{01}}^{{y}} $ mode). Only by changing the length of PMF in single-arm, the optical lengths of two orthogonal MZIs could be adjusted. The length of PMF is select as 15 mm through the analysis of experiment (Fig.4). The single-arm structure generates a Vernier effect, enabling the monitoring of refractive index and temperature by observing the shift of the envelope in the transmission spectrum.The structure of 1.27 mm FMF-15 mm PMF-1.27 mm FMF has been used for refractive index and temperature sensing experiments. The structure eliminates the environmental interference and ensures the measurement accuracy. As the refractive index increasing the transmission spectra has a red shift. The refractive index sensitivity is obtained to be 691.15 nm/RIU with the good linearity of R²=0.996 (Fig.6). As the temperature increasing the transmission spectra has a blue shift. The temperature sensitivity is obtained to be -829 pm/℃ with the good linearity of R²=0.996 (Fig.7). Repeated experiments show that the sensor still maintains good linearity and stability. By comparing with similar works, the proposed scheme is a new perspective using birefringence of PMF, which could achieve Vernier effect in single-arm with polarization interference. The matching of optical path lengths only needs controlling the length of PMF. This compact structure fabricated simply with cutting and splicing that the process is simplified and the cost is controlled. At the same time, the sensor exhibits a good sensing characteristics and stability (Tab.2).A dual-parameter sensor in single-arm structure of orthogonal MZIs based on PMF has been proposed. The structure fabricated by sandwiching a section of PMF between two pieces of FMF. The interference occurs between two orthogonal polarization modes ($ {\text{LP}}_{\text{01}}^{{x}} $ mode and $ {\text{LP}}_{\text{01}}^{{y}} $ mode) and the high-order modes in PMF. Only by manipulating the length of PMF in single-arm, the dimensions of two orthogonal MZIs have been simultaneously adjusted. We analyze the interference mechanism and select the length of FMF as 1.27 mm and PMF as 15 mm. Through demodulating the Vernier envelopes, the sensitivity of refractive index and temperature reached 691.14 nm/RIU and -829 pm/℃. Besides, the structure performs a good stability in repeated experiments. The FMF-PMF-FMF (FPF) structure is capable of minimizing errors induced by environmental fluctuations, which has good potential application in high-precision engineering.