Biomass is a key parameter in fermentation process,directly influencing the performance of the fermentation system as well as the quality and yield of the targeted product.Hybrid soft-sensor modeling is a good method for on-line estimation of biomass.Structure of hybrid soft-sensor model is a key to improve the estimating accuracy.In this paper,a forward heuristic breadth-first reasoning approach based on rule match is proposed for constructing structure of hybrid model.First,strategy of forward heuristic reasoning about facts is introduced,which can reason complex hybrid model structure in the event of few known facts.Second,rule match degree is defined to obtain higher estimating accuracy.The experiment results of Nosiheptide fermentation process show that the hybrid modeling process can estimate biomass with higher accuracy by adding transcendental knowledge and partial mechanism to the process.