This paper contucts 0.6 m icing wind tunnel tests to study the formation law of the runback ice， i.e.， start position， coverage， and type， on a horizontal tail section assembly containing an electrothermal heater under the conditions of a wind speed of 90 m/s， a temperature of -4—-9 ℃，liquid water content（LWC） of 0.45—1.5 g/m³， median volumetric diameter（MVD）20.1—36 μm. Test results show that the initial position of runback ice formation on airfoil surface is obviously affected by the heating power and the incoming temperature. When the heating power or the incoming temperature is lower than certain values， the type of runback ice changes from stream ice to ice ridge. With the increase of heating power or inflow temperature， the initial position of runback ice moves backward. The overflow range of runback ice is obviously affected by LWC and the heating power. The larger the LWC is， the more water is collected， and the wider the overflow range is. The influence of heating power is similar. The increase of heating power will lead to the increase of overflow water， and the overflow range will be wider. The type of runback ice formation is sensitive to the change of MVD. When the MVD changes from 20.1 μm to 3 μm， and the overflow ice changes from typical stream ice to ice ridge.