The photodegradation of methyltins, as environmental pollutants, has scarcely been studied so far because of the shortage of rapid and sensitive speciation methods, even though they have very simple structures. The photodegradation of monomethyltin trichloride (MMT), dimethyltin dichloride (DMT) and trimethyltin chloride (TMT) was studied with our new developed HPLC-FPD hyphenated system, which enables rapid and sensitive detection of methyltins. The half-life times and kinetic rate constants of their degradation at different pH were calculated. The results suggest that MMT, DMT and TMT can be degraded under the UV irradiation rapidly at different pH, with a degradation rate sequence of TMT<DMT<MMT. An unknown intermediary product, which is more stable and has higher concentration at pH 8 for MMT and DMT, of methyltin photodegradation was detected for the first time. This unknown intermediary product was identified as methyloltin with electrospray mass spectrometry, and the possible mechanism was proposed based on the intermediary product. The effects of some environmental parameters such as salinity and humic acid on the degradation rate of methyltins were also investigated. Results suggest that salinity and humic acid have strong effect on their degradation, especially for TMT, which was almost never degraded in the solutions containing NaCl and humic acid.