The invention discloses an all-digital logarithm automatic gain control device and a method thereof. A phase shifter extracts an equidirectional branch data I and a quadrature branch data Q of instantaneous outcoming signal, then a power detector obtains the power of outcoming signal, a logarithmic converter performs the linearizing process to the outcoming signal, compares the results with a system preset parameter b to form an error signal, finally an integrator removes the high-frequency component, an index converter performs the reverse-linearizing process to the error signal, a controllable gain amplifier forms a feedback loop. When the outcoming signal power is larger than the preset value,the gain of the controllable gain amplifier is reduced, and when the outcoming signal power issmaller than the preset value,t he gain of the controllable gain amplifier is enhanced, therefore outputting the digit signal with constant power. The invention avoids the limiting amplitude distortion, considerably reduces the complexity of back-end digital signal processing, enhances the anti-interference and stability of the system and effectively reduces the consumption of hardware system resources.