Natural circulation is an energy transmission mode that depends on the buoyancy force generated by the density difference between cold and hot fluids to drive the fluid circulation flow in a closed system. The natural circulation system has been widely used in nuclear reactors, boilers, solar thermal utilization systems, and many other fields. The research methods on natural circulation loops primarily focused on experiments or numerical simulation of one-dimensional models. However, the one-dimensional model cannot capture the secondary flow and flow stratification, so the three-dimensional CFD simulation is necessary. In this paper, a rectangular single-phase natural circulation loop with a horizontal heater and a horizontal cooler was modeled and investigated numerically. The dynamic flow characteristics of natural circulation under different operating parameters, such as heating power, loop height, and pipe diameter, were studied. The loop system could maintain stable operation at the heating power level of 2 kW or 2.5 kW. At 1.5 kW or 0.5 kW, the loop shows oscillation mode. The heating power and heat sink temperature have a significant impact on the stability of the loop system. A specific heat sink temperature must have a matching heating power range to achieve stable operation. While the flow is stable, increasing the heating power will amplify the mass flow fluctuation and raise the mass flow rate. The inner diameter of the pipe has an essential influence on the flow resistance of the natural circulation system. The larger pipe diameters lead to more unstable flow with a larger mass flow rate. Loops with pipe diameters of 30 mm and below showed stable flow characteristics, while loops with pipe diameters of 35 mm and 40 mm showed unstable oscillation, and the amplitude of oscillation increased with the increase in diameter. The mass flow rates of 20, 25, and 30 mm pipe diameters were 0.045, 0.053, and 0.070 kg/s respectively. The larger the aspect ratio is, the more stable the loop is, and the smaller the mass flow fluctuation is. The higher loop height ensures more stable operation and a larger mass flow rate while producing a slightly wider acceptable power range.