For biomass energy transformation in steam boilers, gasification gas has more advantages than coal or oil, such as cost, environmental protection, and policy benefits. However, we must solve the problem of secondary pollution out of tar in its application. In this paper, firstly we propose an improved technical solution, which can produce high heat value, low tar content biomass gas by updraft fixed-bed biomass gasifier with gas outlet in the middle position, and the tar composition burned directly in boiler furnace with biomass gas. Based on the technical solution, we take 2T/h steam boiler for example to design and calculate the main structure parameters of the gasifier. Next, we provide an intelligent adaptive control on air supply of gasifier according to steam pressure. Finally, the feasibility and efficiency of biomass gas as a new boiler fuel was proved through the test data in practical application cases.