A dual-comb system based on phase locking to ultra-narrow linewidth lasers is studied and implemented in this paper. Two Erbium-doped mode-locked lasers with repetition rates of about 200 MHz and repetition rate difference of about 17 kHz were simultaneously locked to two narrow-linewidth continuous wave lasers at 1 542 nm and 1 560 nm, respectively. Hence, dual-comb with high coherence and stability is achieved. After phase locking, the comb linewidth of a single comb is less than 5 Hz（limited by test condition）, and the relative linewidth of two combs is less than 0.35 Hz. The dual comb system has important application value in high precision spectral analysis, time-frequency transmission, and especially absolute distance measurement.