The principle states of polarization (PSP) alignment in multilevel cascaded semiconductor optical amplifiers (SOA) system are studied based on the relationship between their PSP alignment and Poincare Sphere′ s correlation. Scheme of the all- optical switch is proposed based on nonlinear polarization rotation (NPR) in multilevel cascaded SOAs. An all- optical switch is demonstrated based on NPR in two cascaded SOAs. A light signal of 50 μW is all- optically switched by a square wave control light of 6 mW, and the switching speed is 200 ps. The all- optical switch′ s wavelength band is from 1510 nm to 1570 nm, and its insertion loss is 3.62 dB and power consumption is 12 mW. The all- optical switch has advantages like high switching speed, low insertion loss and power consumption, and good compatibility with optical fiber communication system and will effectively promote the technology development in all-optical switch field and all-optical network field, as well as optical computing field and all-optical signal processing field.