This paper presents a PWPLL (Pulse-Width Controlled PLL) using a variable-length ring oscillator for autonomously tracking PVT (Process, Voltage and Temperature) variations. We fix the ring oscillator control code and the numbers of the reference CLKs and the feedback CLKs are compared, and then the additional Tracking Circuits switch the number of stages for a PWPLL to be locked. The PWPLL using a variable-length ring oscillator is implemented in a 180nm CMOS process. The measurement results of the prototype chip show it achieves 25% increase of the lock range from a conventional PWPLL, and 0.0265mm² layout area along with 2.99ps rms jitter while consuming 4.79mW under a 1.8V supply with 600MHz output frequency.