An indoor event detection system includes a transmitter and a receiver. The transmitter includes a first antenna, and the receiver includes a second antenna, a processor and a memory. The receiver communicates with the transmitter based on a line of sight link between the transmitter and the receiver covering an area to be detected in the indoor space. The processor detects whether an event associated to the indoor space is occurred by obtaining a current CSI from the probe signal, obtaining an amplitude matrix by extracting a plurality amplitudes of a plurality of sub-carriers from the current CSI, applying a statistical operation on the amplitude matrix to obtain a statistical matrix, obtaining a plurality of eigenvalues from the statistical matrix, obtaining a current eigenvalue statistical value according to the plurality of eigenvalues; and determining whether the current eigenvalue statistical value is within the first eigenvalue range.