This paper proposes an internally modulated chirped pulse based direct detection type phase sensitive time domain reflectometer (φ-OTDR) system with high signal-to-noise ratio (SNR). Based on theoretical analysis, we find that the stimulated Brillouin threshold increases with the increasing injection pulse energy, the high direct current component of the detection signal keeps the signal and noise separate, and the SNR can be enhanced. Then a statistical evaluation method is proposed to calculate the SNR of the sensing system. Meanwhile, an internally modulated direct detection type φ-OTDR system based on the distributed feedback laser is experimentally studied. The frequency chirp range introduced by internal modulation is controlled by the tunable filter. Compared with the traditional φ-OTDR system, the proposed system can obtain similar SNR and smaller distribution variance and realize higher measurement reliability. Additionally, The proposed system has the advantages of low cost and small volume, and the tunable filter can be replaced by fiber Bragg grating in practical application. It can improve the flexibility of application and provide a new idea for engineering.