In rubrene-doped organic thin films, the average intermolecular distance between adjacent rubrene molecules was changed by controlling the doping concentration of rubrene. In rubrene: Alq3 composite films, the average distance of doped rubrene molecules was decreased from ~ 1.7 nm down to ~ 1.1 nm when the weight ratio of rubrene was increased from 14% to 50%. The variation of intermolecular distance affected the magnitude of overlap of exciton wave function between adjacent rubrene molecules, hence tuning the strength of intermolecular coupling and leading to different rate of singlet exciton fission between neighboring rubrene molecules. In experiment, the steady-state photoluminescence spectra were firstly measure. And time-resolved fluorescence decay curves of different samples were recorded at room temperature by using transient fluorescence spectroscopy. All the excited fluorescence was from doped rubrene. In theory, based on a three-state reaction model of “S1+S0 ↔ 1(TT) i ↔ T1+T1”, the dynamic process of singlet exciton fission in amorphous rubrene was appropriately described by using three coupled rate equations. All the measured transient decay curves of photoluminescence of rubrene-doped films could be well fitted by the iterative calculation of rate equations. And a detailed analysis about the critical issues which were related with curve-fitting was given. According to the fitting parameters, it was found that the rate of “S1+S0→1(TT) i ” process exponentially decreased with the increasing rubrene distance. Such a phenomenon was not reported before. This is very similar with the previous observations in the process of Dexter energy transfer in which the rate of interaction also exponentially decreased with the increasing donor-acceptor distance. In Dexter process, the energy transfer can be considered as the process of two electrons’ hopping between donor and acceptor molecules. Similarly, singlet exciton fission process also involves two electrons’ hopping between two adjacent rubrene molecules. According to this physical image, the excitation of triplet excitons and redistribution of energy of singlet exciton are accomplished by the formation of an intermediate triplet pair state. The enlarged intermolecular distance would result in exponentially decayed electron-transfer rates, thus decreasing the comprehensive rate constant of singlet fission. Since both of two processes can be explained by “double-electron transfer model”, the similarity in functional relationship could actually be regarded as the first experimental confirmation of “double-electron transfer model” for singlet exciton fission. This is of significant value for clarifying the physical mechanism of singlet fission process. Furthermore, an in-depth investigation about such an exponential relationship between the exciton fission rate and intermolecular distance will be helpful for analyzing the dynamics and key factors of singlet exciton fission process.