The invention relates to a cast aluminum heat exchanger of a gas-fired boiler. The cast aluminum heat exchanger of the gas-fired boiler comprises a front module, a middle module and a back module, wherein a water channel is arranged in each module, a flue is formed between adjacent modules, the front module, the middle module and the back module are formed through die casting, each water channel consists of a main water inlet channel, a heat exchange water channel and a main water outlet channel, the heat exchange water channel in each module is communicated with the main water inlet channel and the main water outlet channel, the main water inlet channels of adjacent modules are communicated and the main water outlet channels of adjacent modules are communicated. Since the cast aluminum heat exchanger is formed by adopting the die casting process instead of the sand casting process, the production efficiency is improved and the manufacturing cost is reduced; since the main water inlet channels, the heat exchange water channels and the main water outlet channels are all built in the modules, the size of the heat exchanger is reduced, the assembling structure is simplified and the installation steps and the material costs of the water channels are reduced.