A soft test method uses for measuring SO3 content during the dihydrate wet-process phosphoric acid production process. The content of dissociative vitriol SO3 is the leading variable in phosphoric acid extraction equipment. The assistant variables are pulp flux, vitriol flux, material slurry flux, fluid level, pulp proportion, material slurry proportion, material slurry liquid phase proportion, material slurry liquid solid ratio, phosphorite CaO content, vitriol proportion and vitriol concentration. The dynamic mechanism soft test model of the wet-process phosphoric acid extraction equipment bases on the leading variable and assistant variables and calculate the SO3 content of the phosphoric acid extraction equipment at real time. At natural produce condition, it can exactly forecast the change current to leading variable SO3, and provide great forecast effect at non-normal condition when the key data take place big fluctuate. It has preferable precision with long observation and provides beneficial direction for the production to avoid the fluctuate of the production and establish automatism forecast control.