Recently, Internet of Things ( IoT) have been applied widely and improved the quality of the daily life. However, the lightweight IoT devices can hardly implement complicated applications since they usually have limited computing resource and just can execute some simple computation tasks. Moreover, data transmission and interaction in IoT is another crucial issue when the IoT devices are deployed at remote areas without manual operation. Mobile edge computing ( MEC) and unmanned aerial vehicle ( UAV) provide significant solutions to these problems. In addition, in order to ensure the security and privacy of data, blockchain has been attracted great attention from both academia and industry. Therefore, an UAV-assisted IoT system integrated with MEC and blockchain is pro-posed. The optimization problem in the proposed architecture is formulated to achieve the optimal trade-off between energy consumption and computation latency through jointly considering computa-tion offloading decision, spectrum resource allocation and computing resource allocation. Consider-ing this complicated optimization problem, the non-convex mixed integer problem can be transformed into a convex problem, and a distributed algorithm based on alternating direction multiplier method ( ADMM) is proposed. Simulation results demonstrate the validity of this scheme.