Topping is an important agronomic measure in tobacco field production, but it is a serious artificial injury to tobacco plants and will cause serious potassium loss. However, the variations of K⁺ flux in tobacco roots after topping and the relationship between K⁺ flux and potassium related gene expression have been rarely studied. In order to analyse the regulatory effect of topping on potassium loss in tobacco, non-invasive micro-test technology was used to study the effects of topping and indole-3-acetic acid (IAA) treatment on plant potassium content, root potassium flux, potassium loss and gene expression in two tobacco cultivars under sand culture conditions. The results showed that topping changed the root potassium flux direction from influx to efflux. Compared with the control, the root potassium efflux rates of EY-1 and Y87 increased by 60.75 pmol·cm^-2·s^-1 and 47.00 pmol·cm^-2·s^-1, respectively, after topping. Topping + IAA treatment decreased the root potassium efflux rates of EY-1 and Y87 by 6.90 pmol·cm^-2·s^-1 and 17.47 pmol·cm^-2·s^-1, respectively, compared with only the topping treatment. Compared with the control, potassium loss of EY-1 and Y87 increased by 60 % and 12 %, respectively, after topping. The application of IAA reduced the potassium loss of the two tobacco cultivars and increased their plant potassium contents slightly, however no significant difference was found. Topping inhibited the synthesis of IAA, promoted the expression of the K⁺ outflow channel gene NTORK1, and inhibited the expression of the K⁺ inner channel gene NKT1. The responses of K⁺ channel genes to topping were different in different cultivars and different parts of the same cultivar. In conclusion, by inhibiting IAA synthesis, topping promotes the expression of K⁺ outflow channel genes, inhibits the expression of K⁺ inner channel genes, increases the outflow rate of potassium ions in roots, accelerates potassium loss, and reduces potassium content in tobacco plants. The application of IAA after topping compensates the potassium loss caused by topping.