Abstract: The aerodynamics sharp decrease, unsteady and nonlinear effects and low prediction accuracy at low Reynolds number result in lots of difficulties and challenges in developing low Reynolds number aircraft, such as near space low speed aerial vehicle and micro aerial vehicle. This paper firstly summarized the region with low Reynolds number and the main problems and challenges of low Reynolds number aerodynamics. Secondly, at low Reynolds number theory aspects, this paper respectively introduced the classical low Reynolds number laminar separation theory, the unsteady flow aerodynamics characteristics at low Reynolds number and trailing-edge laminar separation bubbles. On this basis, with detailed analysis about the aerodynamic differences between classical long laminar separation bubble and trailing-edge laminar separation bubble and the revolution regulation with the changes of the attack of angles and Reynolds numbers, the essence of complicated aerodynamic effects at low Reynolds number has been revealed, for instance, lift nonlinear effect at low attack of angles, secondary deterioration effect of airfoils with decreasing Reynolds numbers. This paper lastly concluded the deve-lopment process of low Reynolds number basic theory, and made outlooks to complicated issues such as transmissions induced by laminar separation and attachment effects after separation.