Abstarct: Due to the high use of thermal desalination plants, there is a great tendency to simulate the behavior of these plants. Most of the researches is focused on modeling steady behavior, but to design control systems and evaluate their performance, we also need to study unsteady behavior modeling. Also, few researches have studied system shut down. In this research, steady and unsteady numerical modeling multi effect of desalination with thermocompressor plant (MED-TVC) on an industrial scale with four effects, a condenser and a thermal vapor compressor (TVC) has been investigated. The variable-step, variable-order method (VSVO) has been used to solve differential equations. Each evaporator is divided into three phases: vapor, tubes and brine, and for each of these phases, the equations of mass and energy conservation in unsteady conditions are written. Results have been validated with real plant data. The variables of temperature, vapor flow rate, brine flow rate and brine level were studied in unsteady modeling in starting, shutting down and steady state conditions. In the following, the effect of fuzzy controller in the system is also shown. The results show that the largest change in brine flow rate after shutting down is in the last effect, which increases by 42%. Also, the biggest change in the height of the brine is in the first effect, which after 800 seconds will reach 11 times the steady state, that will cause the phenomenon of flooding and possible damages. Also, by increasing or decreasing the Motive steam pressure and seawater temperature, the fuzzy controller returns brine levels of the effects to their previous nominal state. This series of results can only be achieved by unsteady modeling of the system by turning it on and off and creating a controller.