ABSTRACT There are insufficient stability problems in overlay construction on tourist highways in mountainous and island areas, especially the pavement with a large longitudinal slope. To elaborate on the instability characteristics of overlay, the construction stage of overlay is divided into loose state, paving stage and initial compaction stage. The moulding methods of specimens during the paving and initial compaction stages were determined. The evolution law of contact-slip characteristics and shear strength of overlay was analysed. The mechanical models of the construction stability of the composite pavements were constructed, instability characterisation methods were proposed and instability characteristics of asphalt overlay during construction were discussed. The results show that the paving state of overlay can be simulated by changing the rolling frequency and the compaction number. There is a risk of slip instability of loose overlay and the longitudinal gradient should be less than 12% to reduce this risk. The critical instability equivalent area ratio and critical slip area proposed can quantitatively describe the shape of the instability body and the instability critical condition. The longitudinal gradient significantly affects the interlaminar instability, but the internal instability mainly depends on the shear strength of asphalt overlay.