This paper chooses 0.1 μm~20 μm monodisperse particles as the research object. We adopted numerical simulation methods to study the transmission characteristics of particles in the straight horizontal sampling tube under laminar flow. Diameters of tube are 4 mm, 7 mm and 10 mm. The lengths of tube are 0.2 m, 0.5 m, 1.0 m and 1.5 m. We got the flow field of the tube and the particle concentration distribution. Through the analysis of simulation results, we obtained the transport efficiency of particles in the straight horizontal sampling tube. Based on the numerical simulation of particle transmission characteristics within the straight horizontal sampling tube, it is found that the particle loss is mainly from gravity and diffusion. Simultaneously, the particle transport efficiency is influenced by particle size, tube length and inner diameter. Under laminar flow, the transport efficiency of 0.1 μm~1.0 μm particles is almost 100%, without the influence of inner diameter, tube length, gravity and Brownian diffusion force. The transport efficiency of 2.5 μm~20 μm particles decreases with the increasing of particle size, tube length and inner diameter. Through the analysis, it will improve the transport efficiency of particles by reducing the length of the sampling tube or the inner diameter on the basis of a flow.The simulation results compared with the theoretical formula, it can be approximated that the transport efficiency of particles is equal to the product of particle transport efficicency respectively under the action of gravity and diffusion.