Air-decking is used in mining for two quite different applications. One is to enhance the fragmentation by amplifying the induced fracturing and the second is for pre-split blasting in which the borehole fracturing is reduced. This paper deals with the first of these effects. A forth coming paper will describe pre-splitting by air-decking. The use of air decks to enhance rock fragmentation and so to reduce explosive costs has become common for quite long time. Although a number of studies has been conducted to verify the advantages of blasting with air decks and to investigate the mechanisms involved, the proposed mechanisms still cannot explain clearly the phenomena observed in practice and the design approach adopted for this kind of blasting is still primary based on rules-of-thumb. In this paper, the theory of shock tubes is adopted to (a) investigate the processes of the expanding detonation products, (b) study the interactions between the explosion products and the stemming or bottom of blast hole, and (c) to decide the distribution of the changing pressure of explosion products along blast hole. Numerical simulation and theoretical analyses are then performed to study the physical process of blasting with air decks. Finally, a reasonable value for the air-decking ratio is decided theoretically. It is shown that the pressure-unloading process caused by the propagation of the rarefaction wave and the reflected rarefaction waves in the detonation products plays an important role in the enhanced fragmentation of rock when blasting with air decks. The unloading process can induce tensile stresses of rather high magnitude in rock mass surrounding blast hole. This favors fracturing of the rock. The reflected shock wave with a magnitude of gas pressure higher than that of the average detonation pressure in a fully charged blast hole acts as the main energy source to break the rock in the air deck and stemming portions. The second and succeeding strain waves induced by the unloading or reloading of the pressure within blast hole also contribute to form the initial fracture network in the rock around the blast hole. It is also revealed that there exists a reasonable range of values for the air-decking ratio. For ANFO, this value varies from 0.13-0.40.