The invention relates to the technical field of biomass detection and discloses an online monitoring method and device for a pre-hydrolysis process of biomasses. The online monitoring device comprises a detection groove, a reference solution groove, a waste solution groove, a light source, a data processing device and an attenuated total reflection probe, which are connected with a reactor; the detection groove is internally provided with the attenuated total reflection probe and a temperature sensor; the attenuated total reflection probe is connected with the data processing device through an optical fiber; a pipeline and/or the detection groove are/is provided with a temperature adjusting device; the reference solution groove and the waste solution groove are connected with the detection groove through the pipeline respectively; the pipeline is provided with a valve and/or a pump. The online detection method comprises the following steps: carrying out fitting modeling according to obtained absorbance and pre-hydrolysis liquid ratio to obtain a detection model; calculating by utilizing the detection model to obtain a pre-hydrolysis yield and cellulose and hemicellulose removing rates respectively. The device and the method have very good practicability and very high accuracy, and have important practical value on online detection and control of hot water pre-hydrolysis technological effect of enterprises.