Focal depth is one of the most difficult seismic parameters to determine accurately in seismology. The focal depths estimated by various methods are uncertain to a considerable degree,which affects the understanding of the source process. The influence of various factors on focal depth is non-linear. The influence of epicentral distance,arrival time residual and velocity model (crust model) on focal depth is analyzed based on travel time formula of near earthquakes in this paper. When wave propagation velocity is constant, the error of focal depth increases with the increase of epicentral distance or the distance to station and the travel time residual. When the travel time residual is constant,the error of focal depth increases with the increase of the epicentral distance and the velocity of seismic wave. The study also shows that the location error perhaps becomes bigger for shallower earthquakes when the velocity is known and the travel time residual is constant. The horizontal error caused by location accuracy increases with the increase of the epicentral distance,the travel time residual and the velocity of seismic waves,thus the error of focal depth will increase with these factors. On the other hand,the errors of focal depth will lead to change of the origin time,therefore resultant outcomes will all change. Focal depth is one of the most difficult seismic parameters to determine accurately in seismology. The focal depths estimated by various methods are uncertain to a could degree, which affects the understanding of the source process. The influence of various factors on focal depth is non- linear. The influence of epicentral distance, arrival time residual and velocity model (crust model) on focal depth is analyzed based on travel time formula of near earthquakes in this paper. When wave propagation velocity is constant, the error of focal depth increases with the increase of epicentral distance or the distance to station and the travel time residual. When the travel time residual is constant, the error of focal depth increases with the increase of the epicentral distance and the velocity of seismic wave. The study also shows that the location error may become bigger for shallower earthquakes when the velocity is known and the travel time residual is constant. The horizontal erro r caused by location accuracy increases with the increase of the epicentral distance, the travel time residual and the velocity of seismic waves, thus the error of focal depth will increase with these factors. On the other hand, the errors of focal depth will lead to change of the origin time, therefore resultant outcomes will all change.