Balance field length is one of the important technical indexes in civil aircraft design and operation, and it has a significant impaction on civil aircraft economy, safety and universality, and also is the basis of determining the aircraft decision speed. This paper does introduce the concept and calculation method of balance field length and the relation between balance field length and decision speed, and list three sensitivity analysis theories. Taking a civil aircraft as an example, research the effects of takeoff weight, airport height, wind speed, temperature, runway slope and other factors on the balance field length, evaluate the sensitivity of different factors to balance field length with the three analysis theories. The results show that the length of balance field and the decision speed increase with the increase of takeoff weight, airport height, tailwind speed and temperature, decrease with the increase of headwind speed. The effect of runway slope on the balance field length is negligible. The positive and negative correlations of those parameters obtained by the three sensitivity analysis methods are consistent, and the sensitivity of the five variables to the balance field length are ranked as follow: takeoff weight>wind speed>airport height>temperature>runway slope.