In this paper, a concise two-state clock model is proposed to describe the behaviour of drifting internal oscillator clocks with non-identical and time-varying frequencies, which provides a simple analytical solution of different coupling schemes' effects on synchronisation performance. The improved Pulse-Coupled Oscillators (PCO) scheme presented in this paper guarantees the synchronisation on non-identical and time-varying clocks. Meanwhile, it differs from classical PCO by scheduling the transmission of Syncs (i.e., the Pulses of clock) at different time slots. This collision-free pulse coupling scheme makes the concept of PCO applicable to wireless sensor networks, as it is practically impossible in realistic wireless networks for multiple oscillators to transmit Pulses at the same time. Furthermore, a proportional controller is adopted to correct the clock, and convergence and stability of the proposed algorithm are theoretically analysed, proved and evaluated by hardware experiments. The experimental results show that the improved PCO scheme can achieve synchronisation with the precision of around 400 μs on internal oscillator clocks of about 350, 000 ppm.