Studies of the changes in the mass and the geometry of the sessile tiny droplet have been performed. The droplet was evaporated on a hot medium with variable temperatures. Based on the “CCR” (constant contact radius) model, a new theoretical model predicting the relationship between the mass and the geometry of the evaporating droplet when ambient temperature is changing, has been built. A solution is given when temperature changing is the linear function of the time. Measurements of the mass change of the desulfurization wastewater droplet in TGA have been conducted. The contact angle, the contact diameter and the height were detected by a visualization method. The results show that the diameter of the 0.5 μL droplet decreases little at the first stage and then plummets rapidly. This trend is rarely affected by the change of the temperature. Also, the TGA results show that, ranging of the setting temperature increasing rate from 15 °C/min to 45 °C/min and of the volume from 0.5 μL to 2.5 μL, the mass change function, G(t), is almost equal to the geometric function, F(θ). This means the theoretical model has great accuracy and validation for the prediction.