An electronic fence at least comprises a laser emitting platform for emitting linear lasers. Each emitting platform is at least provided with a linear laser emitting head. A light curtain formed by the linear lasers emitted by the linear laser emitting heads is roughly perpendicular to the ground, and the light curtain is edges of the electronic fence. Each emitting platform further comprises a power source and a switch. A robot restrained by the electronic fence carries at least one laser signal detecting device and a conical reflector or multipath light guide facing the laser signal detecting device. According to the laser signal detecting device, laser signals irradiated on the outer surface of the robot can be turned and concentrated to the laser signal detecting device through the conical reflector or multipath light guide. Once the robot detects the laser signals through the laser signal detecting device, it shows that the robot is located at the edges of the fence.