Pixel detector has always been the working platform of high-resolution and high-speed particle tracking.In this work,silicon pixel detector based on poly-Si/SiOx passivating contact heterojunction structure was designed.In order to obtain fast response of the pixel detectors, device simulation was performed using Silvaco TCAD.The influence of substrate thickness on the transportation and collection of signal carriers was studied. Meanwhile,the effect of device structure on the breakdown voltage of detectors was also researched.The simulation results showed that a stronger drift field can be built for the thinner substrate given the same bias voltage,thus a better carrier transportation and collection can be achieved,it helps to improve the time response of detectors .A narrow gap between the active region and the first guard ring was helpful for increasing the breakdown voltage.In the case of wide gap between the active region and the first guard ring, the breakdown voltage can also be improved by designing a field plate structure on the edge of the active region.As a result, the pixel detector can work under a large bias voltage and a fast response can be realized.