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The number of negative air ions is one of important indexes of air quality. Most of the current observing instruments for negative air ions are designed and manufactured for indoor use or short time outdoor use. When applying these devices outdoors, some factors as wind will influence the observing results, leading to measurement accuracy decreasing. The principles of negative airion observing instruments are introduced, and its deviations when used outdoors are analyzed. The different structures of the sensor package are designed, and their different impacts on sensors are compared through the computational hydro mechanical methods. The results indicate that the Ttype package has a more even wind field, and better for the packaging of outdoor instruments.