The invention provides a side-entry light guide plate and a control method thereof, a backlight module and a display device, and relates to the technical field of display. In the invention, a liquid medium is filled in the body of the light guide plate, a mirror array is arranged at the bottom of the body of the light guide plate, and light rays totally reflected to the mirror array with an incident angle smaller than a preset angle are led out of a light emitting surface. Between the mirror array and a light incident surface, due to the presence of the liquid medium, part of the light rays incident from the light incident surface are totally reflected in the light guide plate, and part of large angle of view light rays emitted from a light source are reflected off the light guide plate. The mirror array leads the light rays totally reflected to the mirror array with the incident angle smaller than the preset angle out of the light emitting surface, and the light rays whose incident angle is greater than the preset angle continue to be totally reflected in the light guide plate, thereby improving the collimation of the light rays emitted from the light emitting surface, and effectively solving the crosstalk problem between adjacent partitions of the backlight module. Applications to products such as anti-peep and directional backlight modules can be achieved.