Phase sensitive optical time domain reflectometry (Φ-OTDR) has been widely used in various applications for its distributed measurement capability of dynamic disturbance along the entire length of sensing fiber. In our previous research, the capacity of Φ-OTDR sensing system has been enhanced with ultra-weak fiber Bragg grating (UWFBG) array for high-precision quantitative measurement on the external disturbance. Multipoint nε level dynamic strain variation has been fully captured with 2m spatial resolution. However, only 5 identical UWFBGs were used in the demonstration. For large-scale sensing network, the detectable fiber length would be limited by the signal-to-noise ratio (SNR) and the dynamic range of the reflection signal. In this paper, the performance limit of the UWFGB array enhanced Φ-OTDR sensing system has been investigated. A numerical simulation was performed to illustrate the relationships between the SNR, sensing length, spatial resolution and the UWFBG reflectivity. The simulation results have shown that when the reflectivity is relatively low, the noise is mainly contributed from Rayleigh scattering. On the other hand, the multiple reflections will dominate the overall noise characteristic with the growing of UWFBG reflectivity. The requirement of detection dynamic range was also analyzed. Simulation has shown that lower UWFBG reflectivity will lead to smaller signal dynamic range, making the selection of detector more practicable. The analysis result is quite useful for designing large-scale sensing network based on UWFGB array enhanced Φ-OTDR sensing system in practical situation.