Singlet fission can circumvent energy-loss and improve the photoelectric conversion efficiency of solar cells. The excited state dynamics of rubrene analogueue C38H24S2 in toluene solution under different excitation conditions is investigated using femtosecond time-resolved transient absorption spectroscopy. Under the excitation of 550 nm and 365 nm, the excited singlet excitons collide with ground state molecules, and the correlated triplet pairs are generated at 8.2 ps and 3 ps, respectively. However, no independent triplet state signal is observed with the excitation wavelength of 550 nm. While under the excitation of 365 nm, the induced absorption signal of the triplet pair shifts, and the absorption of independent triplet exciton is observed around the detection wavelength of 550 nm. Besides, the singlet fission is observed by using the singular value decomposition method in the transient absorption spectrum with the excitation wavelength of 460 nm.