I presume you meant that the water droplets were on the inside of the incubator? Condensation requires moist gas and a temperature differential. Is there a leak in your incubator? Which direction? Is there a cold air draft hitting your incubator? Is there excessive humidity inside your incubator?

When moist (humid) air comes in contact with a surface of temperature less than the dew point of that moist air, some of the moisture in that air condense on the colder surface in the form of water drops.

I fully agree to Mahoud's explanation. Even if there is s very small leakage at the gasket also condensation will appear. The small airflow will extract warmth from the material it flow's through. The amount of drops is a matter time.

Thank you so much for all your concern. I think I should check the gasket to see if there is any leakage.

To Michael: yes, there is water dew around the inner glass door. I am not sure if there is leakage in my incubator as I have tried to switch the gasket from another one but dew is still exist. There was a cold air draft hitting my incubator but I have changed the direction of the door away from the AC and it is no use. I am not sure what is excessive humidity referring to. Do you mean I should not fully fill the water tray?

All good answers above; probably a small leak. By 'excessive,' all I meant was the humidity (water vapor content) inside the incubator significantly in excess of what is required? Partially filling the water tray will probably not mediate the humidity because the water and its interface area will still be present. However, the time it takes for the water to evaporate will decrease. Good luck.

I agree with the above. Humidity is required in the incubator, but this is going to condense if the cold air conditioning is hitting it. Are there dampers on the AC vent that can be adjusted, or a damper can be added. The air flow should not hit the incubator. Also is there an exhaust fan, there should be, that might help too.