Many animals, such as honey bees and tarantulas, can navigate with polarized light, the key of which is to extract compass information from skylight polarization pattern. Many groups have conducted research on bionic polarization sensors and obtained good orientation results in the open-sky circumstances. However, if the sky is obscured by leaves or buildings, the skylight polarization pattern will be greatly affected. This paper presents an unsupervised method for polarization navigation when the sky is partly blocked. First of all, we introduce the core components of polarized light compass and the measurement method of skylight polarization pattern. Then, an unsupervised method is used to extract the sky region according to the single scattering Rayleigh model. Finally, we calculate solar meridian vector and heading angle using pixels of sky region. Results show that polarization navigation, featuring high anti-interference and no accumulation error, is suitable for outdoor autonomous navigation.