In order to reduce the cost of IOTA system, an IOTA node cost optimization mechanism (CO-IOTA) is proposed based on Lyapunov optimization theory. The key factors affecting the cost of IOTA nodes are analyzed and evaluated by using Lyapunov optimization theory, and the message acknowledgement rate threshold of IOTA nodes is derived. By adjusting the threshold, an algorithm is proposed to effectively reduce the cost of IOTA nodes. The IOTA system with 100 nodes was simulated in Matlab R2022a, and 3 nodes were randomly selected for comparison before and after cost optimization. The experimental results show that the CO-IOTA mechanism can further reduce the cost of IOTA nodes.