This work proposes an original innovative in-house device able to characterize the evolution of viscoelastic properties of lignocellulosic products during hydrothermal pretreatment (water-saturated conditions up to 190 °C). The main objective is to assess the cooking phase before steam explosion. We designed and built a novel experimental device for analyzing the dynamic mechanical analysis of the biomass soaked in water or acidic water. The device allows the sample to be loaded/unloaded during the test with an accurate measurement of deformation and force despite the severe conditions. This system can perform the mechanical test on immersed samples under controlled pressure (up to 1.5 MPa) and temperature (up to 190 °C). Experiments were performed with poplar via harmonic and periodic tests. The experimental data collected on poplar allows the characteristic time constants of the thermo-chemical reactions to be obtained as a function of the operating conditions.