There exist three typical profiles of sediment deposition in reservoir, among which delta profile is the most general one. Based on the nonequilibrium transportation equation, it is verified in theory that the delta formation is the tendency of reservoir sedimentatiom. A general mathematical model of reservoir sedimentation and river fluvial established by the author is used to simulate the entire process of the formation and development of delta, thus its characteristics and mechanism involved are clearly exposed. A series of theoretical results are presented, including:1. deposit percentage of delta, which is dependent on the grade curve of suspended load during sedimentation; 2. equations of gradient and longitudinal profile of top-slope of delta. The former belongs to the second order polynomial and the later to the third order polynomial, both of which are concave-up curves, meaning that the curve slope is decreased along the flow direction and the top-slope of delta is not a linear curve as shown in some previous papers; 3. expressions of water level upon the delta top-slope and the slope of water level, which are also third order and the seceond order polynomials respectively and the concave-up curves as well. But the decreasing of water level is smaller than that of the delta bed and the water depth is increased along the flow direction; 4. expression of the length of the front slope, the end of which is the location where the deposit percentage is just equal to that of the delta; 5. deposit percentage of the top-slope, which is derived from the calculation of profile; 6. variation of size distribution of suspended load at each section of the delta and the grade curves of deposit at the front and top-slope respectively. All these theoretical results are verified by the measured data at Guantixig Reservoir, a typical depositional deltaic one, showing a satisfactory agreement.