A distributed fiber sensing system based on weak fiber Bragg gratings (wFBGs) enhanced phase-sensitive optical time-domain reflectometry (Φ-OTDR) has improved signal-to-noise ratio (SNR) and sensitivity. The effects of wFBGs on a Φ-OTDR system are investigated theoretically and experimentally. The SNR improvement induced by wFBGs, which have higher reflectivity compared with the Rayleigh scattering, has been analyzed in systems with different laser source linewidths. The experimental results show that Φ-OTDR assisted by wFBGs with −35-dB reflectivity has a lower noise level and its SNR is improved by 27 dB. By comparing laser sources with different linewidths, it is proved that the enhanced system no longer requires an ultranarrow linewidth laser. The SNR in different locations of a 2.4-km-wFBG-array remains at a high level.