With the further development of deep oil and gas resources, the deep and ultra-deep wells are used widely, which make lateral vibration of bottom hole assembly (BHA) more violent. Lateral vibration greatly restricts the service efficiency and life of downhole devices. To study the main controlling factors and formation mechanism of lateral motion states in slightly deviated wells, the numerical simulation and indoor experiment were conducted. A drillstring dynamics model with three-dimensional beam elements was built, which combined the advantage of the finite element method and the beam-column theory. The simulation and experiment were carried out to analyze the motion states and motion laws under different drilling coefficients. Four kinds of conventional motion states were classified, and the physical characteristics, forming conditions, and identification methods were given. Four main controlling factors of motion states were summarized, and the formation mechanism was discussed through the principle of least energy consumption and least work consumption. The researches show that the numerical and experimental results are in good agreement. Meanwhile, well inclination, rotational speed, and friction coefficient have significant effects on motion states. The states contain forward whirl, random motion, and backward whirl in vertical wells, and circular arc swing in slightly deviated wells. Moreover, radial component of gravity promotes the formation of circular arc swing, centrifugal force promotes forward whirl before drillstring contacts wellbore and promotes backward whirl after that, tangential friction promotes backward whirl, and internal friction promotes forward whirl. Finally, the paper holds that BHA in vertical wells should operate in the motion state with least output power of the system and also operate in the motion state with least input power of external force during steady motion. The study can lay a theoretical foundation for predicting, identifying, and controlling motion states in drilling field to reduce vibration and improve drilling efficiency.