Based on the diffusion dynamics, a new theoretical model is established to investigate the dynamic properties of a polymer mixed brush (PMB) in the protein adsorption/desorption process. The theoretical model considers the adsorption of proteins by one polymer chain (P-polymer chain) and the desorption of proteins by another polymer chain (N-polymer chain) in a PMB, as well as the time delay between adsorption and desorption. The dynamic properties of protein adsorption/desorption by a PMB depend on not only the chemical and physical properties of polymer chains, but also the microenvironment (density of protein in the solution and protein diffusivity) of the PMB. In order to describe the different chemical and physical properties of polymer chains and microenvironments in PMB, we take different model parameters, and obtain partial adsorption/desorption, complete adsorption/desorption and periodic adsorption/desorption of proteins by the PMB. By analyzing the process of protein adsorption/desorption in a PMB, we find that the microenvironment has an obvious influence on the adsorption and desorption of protein by the PMB. It is also shown that the adsorption of protein and the desorption of protein by the PMB have a stable and invariable periodic cycle when an alternating electric field is applied. The average adsorption capacity and the average desorption capacity increase in comparison with those when no electric field is applied. A stable alternating electric field enables the PMB to exhibit stable periodic dynamic characteristics in the dynamic process of protein adsorption and desorption. Our theoretical results are consistent with the experimental observations. Based on this, it is predicted that an external electric field can realize multiple cycles of protein adsorption and desorption by PMB, which provides necessary references and useful insights into controllable protein adsorption/desorption by the PMB in the practical applications.