Recoverable automatous sonde (RECAS) is an effective method for clean observation and sampling of polar subglacial lakes. But how to control the verticality of RECAS during drilling is a technical challenge that needs to be solved to accurately penetrate into the target area of the subglacial lake and achieve precise sampling of the subglacial lake water. Therefore, a mechanical model and a formula for calculating the inclination angle of RECAS during drilling are established. Taking the parameters of  RECAS-500  as an example, the influence mechanism of RECAS structural parameters, drilling parameters, and uneven bottom melting on the initial and maximum inclination angles of the sonde is quantitatively analyzed. The result shows that the maximum inclination angle of RECAS increases with the decrease of drilling speed and RECAS length-diameter ratio. When the drilling speed is less than 0.8 m/h and the RECAS length-diameter ratio is less than 40, the trend of increasing the maximum inclination angle is obvious. The influence of ice temperature on the maximum inclination angle of RECAS is relatively small, with an overall fluctuation within 0.3°. The drill load directly determines the inclination angle of RECAS in the borehole when the RECAS structure parameters are determined. To control the inclination angle of RECAS within 1°, the drill load should be controlled within the range of 200-600 N as much as possible, and the maximum drill load should not exceed 1 000 N.