The effects of double pulse delay time and ambient pressure on femtosecond laser ablation of Si are studied by means of photoluminescence and morphology analysis. Detailed scan of double pulse delay is performed at vacuum (2Pa) and atmospheric pressure, and detailed scan of pressure is performed at three double pulse delay times of 0.2ps, 53.13ps and 106.47ps. It is found that, at various fluences, photoluminescence intensity and morphology change as functions of both double pulse delay and ambient pressure. Especially, the shape of the splashing droplets also changes at different experimental conditions, indicating higher or lower sample temperature. The observations are explained by the efficiently energy coupling between the second pulse and the liquid layer produced by the first one, and the pressure dependent energy coupling between plasma and liquid phase as well as ambient gas.