This paper ,through experimental tests and theory analysis, studies the air flow characteristics and further more deduces the calculation formula of the coefficient (S) with one beam upright the airflow direction in the building insulation interlayer.The results show that when 0.3 ≤ m ≤ 0.7 , 0.2 ≤ v ≤ 1.0m/s,4.08×103 ≤ Re ≤ 2.04×104, the coefficient(S) can be calculated by (S) =(1-1/K)2, the experimental results are in good agreement with the calculation results conducted from calculation formula the by the calculation formula of the coefficient (S).Thus it is safe to calculate the coefficient(S) with the formulas.