Piezotronic strain sensors have drawn a lot of attention since the piezotronic theory was established. In this work, we developed a flexible piezotronic strain sensor based on an indium-doped ZnO nanobelt, of which the top surface was the monopolar surface. By connecting two electrodes with the two ends of the top surface of the nanobelt, the strain sensor was constructed. Compared with a nanorod/nanowire based strain sensor, this monopolar surface device avoids the need to identify the polar direction. Under strain, a static potential with the same value and polarity was generated by the coupling effect of the piezoelectric effect and the Poisson effect. This induced piezopotential influenced the Schottky barrier heights at the interfaces of both the source and drain electrodes, resulting in current changes with the same trend at forward and reverse biases. By applying a series of periodical strains, the sensor showed clear, fast and accurate current responses. The gauge factor achieved for compressive strain was 4036. This type of piezotronic strain sensor with a polar surface facing upward presented a high performance and easier fabrication, showing promise for applications in electrical mechanical sensors and MEMS.