1/3 continues phase chirp keying（1/3-CPCK）is proposed as a new narrow-band digital modulation scheme. 1/3-CPCK employs sectional type chirp signal as modulated signal waveform samples, of which the phase is continuous, the frequency is smooth and the energy is highly concentrated. This technique can be used in narrower band for high-speed data communication. The mathematical model of 1/3-CPCK is set up in this paper, the similarity of modulated signal waveform samples is analyzed, and the BER performance under AWGN based on coherent demodulation is presented. Theoretical derivation and simulation indicates that the waveform of 1/3-CPCK modulated signal is smooth and it has high energy concentration. When transmission bandwidth is absolutely limited, bandwidth efficiency is 1 bps/Hz and E_b/N₀ is 13.5 dB, BER of 1/3-CPCK Communication system can reach 10-4 by setting proper modulation factor.