The paper deals with the development of an adaptive digital system of modal control of technological processes. The analysis of a typical adaptive digital control system with an adjustable model is carried out, in which optimization of digital regulator settings is performed at each quantization tact according to the adaptable model of the object, which leads to a significant increase in the computational load on the system. Determination of the optimal tuning parameters of the digital regulator is carried out by the criterion of ensuring the necessary margin of stability of the system. With this purpose mathematical and software research of stability margin at drift of system parameters of variability of external conditions is developed. The connection between the stability margin and dynamic characteristic of the closed-loop control system is established. Synthesized adaptive digital system of modal control providing sufficient stability margin, given stability margin, differs from the typical adaptive system by the possibility of inclusion of the analysis block, designed to assess the stability margin, as well as to determine the moments of optimization of the tuning parameters of the digital regulator. The stability margin of the control system is set according to the requirements for specific technological processes. Conducted studies with different parameters of models of the control object showed the effectiveness of the proposed adaptive digital control system of a dynamic object relative to the known.