How to reduce the Root mean square error I have dataset which describe "how many passenger arriving in some airport " and I would like to predict how many passenger arriving in monthly bases for next year. The features that I have is the following : 

year,
month,
airport,
number of passenger (monthly)

In the data, three out of of 50 airports are usually have huge passengers arriving. 

I have used random forest classifier but the issue that i'm encounter is I have RMSE is high. As a result i see huge different between the actual value and predicting value. How to fix this issue ?
The problem is that you are using a classifier. You should use RandomForestRegressor or other type regressors. 

Those three airports seems like leverage points. Check this wiki topic. You can log transform your target values or simply remove those 3 airports from your training set.

But still you have very less observations. You may want to add new data to your set. After train test split 35 observations for train 15 for test seems not so satisfactory.