The strength, water sensitivity and structure of the HAC/PVA based MDF composites were studied, in which the cement phase was partially replaced by fly ash. The results indicated that the addition of ground fly ash to the MDF composites improved the pore structure and water sensitivity of the MDF composite but obviously decreased the mechanical properties. By the surface treatment of fly ash and cement with silane coupling agent, the flexural strength of the MDF composites containing fly ash can be significantly increased and the water sensitivity is further improved.