The invention discloses an LED (Light-emitting Diode) constant current drive system and a constant current control circuit thereof. The constant current control circuit comprises a voltage loop circuit and a current loop circuit. According to the LED constant current drive system and the constant current control circuit thereof provided by the invention, when an analog dimming voltage signal is larger than first threshold voltage, a loaded current is kept in constant through the duty cycle of a square wave signal adjusted and output by the voltage loop circuit; when the analog dimming voltage signal is smaller than the first threshold voltage, the loaded current is kept in constant through the pulse frequency of the square wave signal adjusted and output by the voltage loop circuit; the dual-mode automatic switching control is realized, and under the normal working condition, the working mode is pulse width modulation; under the low load working condition, the working mode is automatically switched to pulse frequency modulation, so that the overall working efficiency of the system is effectively improved. In addition, through the current loop circuit, the loaded current is adjusted to be kept in constant according to the analog dimming voltage signal and the loaded sampling current; the response speed is improved; the two loop circuits are controlled at the same time, so that the control accuracy, the response speed and the system stability are improved.