Connected and automated technology enables multiple trucks to platoon with a short spacing, promoting the formation of the truck barrier effect in the diverging area, thus having a significant impact on traffic flow. In the connected and automated environment, quantifying the discriminatory conditions of the truck barrier effect and exploring the traffic impact are important to improve the level of service in diverging areas and guide the development of platooning technology. Based on the Simulation of Urban Mobility (SUMO) simulation platform, the influence mechanisms of macro parameters such as arriving flow rate of departing vehicles (AD), arriving flow rate of going-straight vehicles (AS), and truck proportion (TP) and micro parameters such as time headway (TH) and platoon length (PL) on the truck barrier effect are analyzed. Macroscopically, different combinations of AD, AS and TP are derived for the truck barrier in the diverging area to form and as the TP increases, the departing demand and going-straight demand required for truck barrier effect occurrence decreases. Microscopically, TH ≤2s and PL>5 are the conditions for the truck barrier effect in the diverging area to appear. In addition, the truck barrier will increase the lane change rate and speed standard deviation, which will negatively affect the safety and stability of traffic flow. With the increase in traffic demand, the negative impact of the truck barrier becomes more significant.