The invention discloses a biomass gas steam generator. The generator comprises a pre-combustion chamber, a mixing chamber, a lean combustion chamber, a smoke tube, a smoke returning chamber, an axialblade combustor and an ejector; lithium bromide dilute solution or water is introduced through an introduction opening, the lithium bromide dilute solution or water is heated through the lean combustion chamber, a second return smoke pipe, a third return smoke pipe, a lower middle smoke returning chamber, an upper middle smoke returning chamber and a pipe plate to form saturated steam; lithium bromide concentrated solution is introduced into a low-pressure generator from a outlet opening through a liquid bag; the saturated steam is led out from the outlet opening, secondary air is introduced from a secondary air inlet opening, hot air formed by heating is mixed with the weak reducing flue gas at an outlet of the pre-combustion chamber in the mixing chamber through the ejector to form high-temperature high-speed lean air, then the high-temperature high-speed lean air is sprayed into the lean combustion chamber, and is mixed with secondary biomass gas to realize lean combustion. The biomass gas steam generator has the advantages of rich combustion and lean combustion, the energy consumption of a fan is low, the radiant heat flux is high, the temperature field uniformity is good, thecombustion is stable, the Nox generation amount is low, the greenhouse gas emission is low, and the structure is compact.