Based on the fundamental approximation equivalent circuit of a single cell LLC resonant converter, this paper has developed the equivalent circuit of the interleaved LLC resonant converter, with its primary side connected in series and its secondary side connected in parallel. The load sharing characteristics of the series-parallel connected interleaved LLC converter is then analyzed and a current sharing mathematical model is developed to illustrate the current sharing principles. A case example of a 300/12-18V, 2.5kW, 500kHz interleaved LLC resonant converter is designed and simulated to verify the proposed load sharing model. The established model's error analysis is conducted and the developed model is proved to have relatively high precision in current continuous mode and satisfactory precision in current discontinuous mode. The Monte Carlo analysis is executed to get an overall review of current sharing among different frequencies and the Worst Case analysis is executed to reveal the sensitivity of different influencing factors. The paper provides innovative and valuable references for the dimensioning of series-parallel connected interleaved LLC resonant converters.