Mini-LED backlight can realize local dynamic dimming through refined partitions, which is a research hotspot of current Liquid Crystal Display(LCD) backlight technology. This paper aims to reveal different impact on the optical performance and system architecture of the backlight module by different Mini-LED local dimming unit types. We establish three different optical models of the Mini-LED local dimming backlight unit, namely the direct-lit dimming unit, edge-lit dimming unit, and the corner-lit dimming unit proposed in this paper. Through optical modeling and simulation tracing for the optical models with Mini-LED light sources in different structures, the uniformity and light efficiency of the dimming unit are compared. The results show that the light uniformity is 6727% and the light utilization rate is 86.67% of the corner-lit dimming unit without optical films. The light uniformity is 84.70% and the light utilization rate is 92.10% of the corner-lit dimming unit with optical films. The optical performance has improvement in different level compared with the edge-lit local unit and direct-lit local unit. This results have important theoretical guiding significance and application value for the design of Mini-LED backlight units.