Abstract: It is urgent to propose a new technique for the in-situ evaluation of collapsible loess, as the existing methods are time- and cost-consuming, and may lead to inevitable sampling disturbance. The gravimetric water content and dry density of loess are computed by the dielectric permittivity and electrical conductivity measured by the time-domain reflectometry (TDR) in the field. The collapsibility of loess is evaluated using the computed gravimetric water content, dry density of loess and physical properties of soil (i.e., specific gravity, plastic and liquid limits) through empirical relationships. Comparing with those obtained by the oven-dried method, the dry density and gravimetric water content of loess by the TDR are within a relative error of ±6% and an absolute error of ±0.02, respectively. On the other hand, empirical relationships relating the gravimetric water content, dry density and soil physical properties to the collapsibility show a good performance to evaluate collapsible loess. Moreover, the strong and medium collapsible loess may also be characterized. This study extends the techniques of site investigation in loess regions and provides beneficial explorations for the in-situ evaluation of collapsible loess.