A method for heavy oil lightening and synthesis gas production and a device thereof are provided, where the method uses a cracking/gasification coupled reactor, which internally has a cracking section and a gasification section that communicate with each other, and includes the following steps: feeding a heavy oil material into the cracking section to implement a cracking reaction, to produce a light oil gas and a coke; the coke being carried by the coke powders and descending into the gasification section to implement a gasification reaction, to produce a synthesis gas; at least performing a first stage gas-solid separation, collecting coke powder particles and dividing them into two parts; performing an oil and gas fractionation on a purified oil and gas product output by the gas-solid separation system, and collecting a light oil product and a synthesis gas product.