The crystallization behavior of desulfurization product is directly related to its high-temperatureresistant ability. Effects of the additive on the sulfur-fixation efficiency of the Ba-sulfur-fixation agent and also on the crystallization behavior of the sulfur-fixation product were studied when CaCO3 and BaCO3 were used as the desulfurization agent and MgO and SrCO3 used as the assistant sulfur-fixation agent. The result shows that increase of sulfur-fixation capability for the additive is not owe to their directly react to form sulfate or interact with CaCO3 and BaCO3 to form composite mineral heat-resistant in high temperature, but owe to their activation to sulfur-fixation reaction of the sulfur-fixation agent.