Energy, precision, and multi-functionality have become primary considerations in incubators. For example, if a part of an apparatus must be insulated, while other parts do not, then the specific part can be placed in the multi-functional incubator proposed in this paper to avoid wasting energy. The proposed incubator is composed of various parts including a platform, which is provided for debugging parts to meet the working accuracy of parts. The light windows are adjustable in height to accommodate different heights of light. The incubator is black to prevent stray light in optical experiments. This design can not only insulate but also debug optical or non-optical parts according working accuracy of parts. In this paper, an example incubator is used for an optical experiment was given and the incubator was optimized several times. The temperature fluctuations of the box of the third optimized incubator reached 0.045 degrees C. To reduce the temperature fluctuations, the relationship between the ambient and target temperature as well as the temperature fluctuations of the box were analyzed, and a formula was proposed. A further experiment provided evidence for this relationship. Based on the formula, the SLITA (small optimized incubator placed in large optimized incubator with the same target temperature) method for improving insulation accuracy was further proposed, and an incubator was designed using this method. The temperature fluctuations were 0.000014 degrees C by simulation, which was reduced 98.6% compared to previous incubators.