Nonlinear polarization rotation (NPR) technique has been studied for several decades to investigate the ultrafast pulse in both scientific and industrial fields. However, some drawbacks such as sensitive to the environment, noncompact, difficult to reach all the polarization states limit the real applications of this methods. In this work, an improved NPR technique, with polarizar-polarization controlar-polarizar structure, has been proposed for the first time. The new structure is relatively more simple to reach all the polarization states and remains much more stable states than traditional NPR technique. We proposed all-normal-dispersion Yb-doped fiber laser and rectangular-shaped spectrum is obtained both experimentally and theoretically by improved NPR technique. The achieved stable pulse duration and pulse period are 11.69 ps and 37.2 ns, respectively. Signal-to-noise ratio is 64 dB at 26.9 MHz, thus the proposed fiber laser has very high stability. Besides, we investigate the output spectrum and pulse shape based on dispersive Fourier transformation, getting the right triangle and ordinary triangle spectrums that are symmetric with the corresponding pulse shapes. © 2022 Wiley Periodicals LLC.