The invention provides a window grating security system, which comprises a strip light transmitter and a strip light receiver, which are vertically arranged at the two sides of an indoor window respectively. The emission direction and receiving direction of the strip light transmitter and the strip light receiver both penetrate through the window glass and face to the outer side of the window. The outer side of the window is provided with a reflection mechanism. The reflection mechanism enables signal light emitted by the light transmitter to form a semi-encirclement ring for the window outdoors, and the signal light is reflected to the light receiver; the light receiver is coupled to an alarm device; and when the light receiver receives no signal light, the alarm device is triggered to give an alarm. The system can reliably detect abnormal invading cases outside the window, and the security mechanism thereof cannot be damaged at the outer side of the window.