Embodiments of this disclosure provide an apparatus and method for monitoring a polarization dependent loss, a receiver and a communication system. The apparatus for monitoring a polarization dependent loss includes: a first processing unit configured to process received optical signals, to obtain a first correlation matrix of noise signals in the received optical signals and a second correlation matrix of second signals or other signals in the received optical signals other than first signals or spectral feature signals having predetermined spectral features; a matrix subtraction unit configured to subtract the second correlation matrix by the first correlation matrix, to obtain a third matrix; and a calculating unit configured to perform singular value decomposition on the third matrix, and calculate a polarization dependent loss according to a result of the singular value decomposition. According to the embodiments of this disclosure, the polarization dependent loss may be calculated more accurately.