With the market demand for constant temperature, low cost and environmentally friendly bathtub is increasing. Our goal on this paper is to establish a water temperature model to meet the needs of users’ comfort and design a water temperature bathtub without external heating system. After mathematically analysis, we quantify the factors that cause heat dissipation and heat absorption in the bathtub. Based on the Energy Conservation Law, we set up an optimization model of two factors in time and space. We ignore the water temperature stratification, so we transform the three-dimensional problem into a two-dimensional problem to simplify the model. Then, we use MATLAB to simulate the heat distribution of the bathtub temperature field to compare the different locations of injected water. By comparing, we think the optimal strategy is injecting water from all around of the bathtub. Finally, we test the correctness of the water temperature model. We take advantage of the heat energy distribution in the whole bathtub simulated by MATLAB to analyze the influence of the human state on the water temperature in the bathtub. As can be seen in the simulation, the human movement will promote the dissipation of heat and affect water temperature distribution of the bathtub which proves the reliability of the water temperature model. Compared with the bathtub on the market, on the premise of ensuring the comfort of the user, our bathtub and the proposed strategy are of great significance in reducing the cost and saving water resources.