Nowadays, blockchain as emerging computer technology is gradually applied to the Internet of Things (IoT) industry. However, high traffic load and network congestion have become the main obstacles to the integration of IoT and blockchain, thanks to the vast amount of IoT devices as well as the considerable information redundancy of blockchain. In IoT-blockchain, the number of IoT devices and the blockchain network scale are supposed to establish an appropriate balance. Therefore, building a precise traffic model between the blockchain and IoT devices is crucial for designing and optimizing an IoT-Blockchain network. This paper proposes an innovative traffic model of the IoT-blockchain system, which describes traffic of blockchain nodes under the different sizes of IoT input and network scale. We implement a lightweight Bitcoin-like blockchain based on PoW and connect with several IoT simulators to conduct various network traffic experiments. Our mathematical model and experimental results demonstrate the relationship between self-similarity, network traffic, blockchain scales, and the number of IoT inputs.