The invention provides a polarization-maintaining flat 
optical fiber coupling ball microscale sensor based on polarization state detection and belongs to the technology of manufacturing and measurement of precise instruments. The polarization-maintaining flat 
optical fiber coupling ball microscale sensor comprises a 
laser, a semi-transparent semi-reflective 
prism, a quarter 
lambda wave plate, a probe, a polarization state detection device A, a polarization state detection device B and a computer, wherein the probe is composed of a flat 
package layer polarization-maintaining 
optical fiber and a 
coupling ball, and the 
laser, the semi-transparent semi-reflective 
prism, the quarter 
lambda wave plate and the probe are arranged in sequence. Emergent light of the 
laser is reflected and transmitted by the semi-transparent semi-reflective 
prism, and reflected light enters the polarization state detection device A; transmission light enters the probe, reverse transmission of a transmission 
light beam is achieved after the transmission 
light beam passes through the coupling ball, the transmission light exits from the probe, and emergent light passes through the quarter 
lambda wave plate, is reflected by the semi-transparent semi-reflective prism again and then enters the polarization state detection device B. When the difference value between the polarization state detected by the polarization state detection device B and the polarization state detected by the polarization state detection device A is changed, it can be determined that the probe is in contact with a detected hole. The polarization-maintaining flat 
optical fiber coupling ball microscale sensor is high in precision and high in speed.