The shape of ice-water interface has significant impact on the form and law of ice growth. However, knowledge about the form of icing in the supercooled water is insufficient. This paper investigates the unsteady ice growth on a planar solid substrate in supercooled water. An experiment is designed to trigger icing on a solid substrate and observe the whole icing process from two directions simultaneously. An evolution of ice growth that consists of three stages is observed. Based on the experimental results, a theory is presented to explain the mechanisms of ice growth and stage conversion of in the icing process. This theory includes the equations of ice growth velocity in three stages and the criterion of ice-water interface stability in an unsteady temperature field, which determines the icing form and rate. A multi-stage icing model is presented for icing prediction. The calculated results using this model could predict the conversion of icing forms and fits well with experimental results. Therefore, the change in the form of icing significantly impacts the ice accretion process on a solid substrate, such as aircraft or outdoor devices. (C) 2015 Elsevier Ltd. All rights reserved.