A ring oscillator clock automatic synchronization method of a panel driving circuit includes steps of: when a vertical blanking interval happens, a master driver generates a pulse signal to slave drivers respectively. A pulse width of the pulse signal equals to N times of a master ring oscillator clock, wherein N is larger than 0. When a slave driver receives the pulse signal, the slave driver uses its slave ring oscillator clock to count the pulse width of the pulse signal to obtain that the pulse width of the pulse signal equals to M times of the slave ring oscillator clock, wherein M is larger than 0. The slave driver compares M with N and automatically adjusts the slave ring oscillator clock according to the comparison result to make it achieve synchronization with the master ring oscillator clock.