The challenges facing high-performance lithium-ion batteries are not only chemical, but also key mechanical problems. Herein, we perform an experiment to investigate the crucial but difficult task of measuring the elastic modulus and total stress evolutions of a bilayer Si-composite electrode during electrochemical processes. Electrode bending deformation is measured in-situ through an optical system. And four mechanical models featuring Li concentration-dependent elastic modulus are used to quantitatively convert curvature into elastic modulus, normal stresses in electrode material and in current collector, and shear stress along interface. Experiments show that the elastic modulus and the three stresses decrease and increase nonlinearly with capacity, respectively, and all demonstrate hysteresis loops during lithiation and delithiation processes. The significant softening of electrode material indicates that the elastic modulus is not linear with Li content. And further discussions show that all the three stresses are significantly affected by elastic modulus, exhibiting a degree of relief similar to material softening. These results guide the mechanical modeling and provide experimental data for mechano-electro-chemical degradation for Si-composite electrode.