The invention discloses a T5/T8 fluorescent light conversion device comprising a first conversion seat with a concave interface arranged at the inner side and a second conversion seat with a convex joint arranged at the inner side. The T5/T8 fluorescent light conversion device is characterized in that first pins which are mutually communicated are arranged at the outer side of the first conversion seat; two second pins which are mutually communicated are arranged at the outer side of the second conversion seat; a first input connecting end communicated with the first pins is arranged at the inner side of the first conversion seat; a second input connecting end communicated with the second pins is arranged at the inner side of the second conversion seat; the first input connecting end is matched with a fire line input end or a zero line input end of a T5 energy-saving fluorescent light; and another fire line input end or another zero line input end on the T5 energy-saving fluorescent light, which has different polarity from that of the first input connecting end, is matched with the second input connecting end. The T5/T8 fluorescent light conversion device has the advantages that the T5 energy-saving fluorescent light can be conveniently applied to a general T8 fluorescent light, and the aims of saving energy and decreasing consumption can be achieved without large-scale engineering modification.