Distributed acoustic sensing (DAS) systems based on coherent optical time-domain reflectometry (COTDR) can obtain vibration information along the fiber length by demodulating the phase information from the backscattered Rayleigh signal in optical fibers. The phase demodulation algorithm is a key technology for phase extraction in DAS systems, and different algorithms exert varying effects on the demodulation results.This study investigates the impacts of two in-phase quadrature (IQ) demodulation algorithms, i.e.,differential cross-multiplying (DCM) and arctangent (Arctan), on the demodulation results. First, a theoretical analysis and numerical simulations based on DCM and Arctan are conducted. Subsequently, a COTDR-based DAS system is constructed to collect data, which are processed and analyzed using both algorithms. Experimental results indicate that the peak values of the demodulated signals obtained using the two algorithms differ by 0.04 rad, with an observed phase shift of 0.01 rad. In addition, based on the different characteristics of the two algorithms, recommendations for their application scenarios are provided.