A movable carrier auxiliary system includes a state detecting device, a braking device, an environmental detecting device and an emergency brake controlling device. The state detecting device detects a movement state of a movable carrier. The environmental detecting device includes at least one image capturing module and an operation module. The brake controlling method thereof includes receive the movement state detected by the state detecting device, and deriving an operating distance based on the movement state with the operation module; capture the environmental image with the image capturing module; determine whether there is an obstruction within the operating distance based on the environmental image; automatically actuate the braking device with the emergency brake controlling device when there is the obstruction within the operating distance in the environmental image, thereby to stop the movable carrier within the operating distance.