The 6xxx series aluminum alloys (Al-Mg-Si(-Cu) alloys) are widely used for the industrial applications in the lightweight construction, automotive and architecture because of their light weight, medium to high strength, excellent formability and good corrosion resistance. It has been reported that trace Sn addition can accelerate ageing kinetics and increase peak hardness of Al-Mg-Si alloys when ageing at high temperatures (>210 ℃). However, the mechanism about it has not been investigated comprehensively yet. For Mg-excess Al-Mg-Si alloys, when aged at 250 ℃, the alloys are hardened by the β'-precipitates. While after applying natural ageing prior to artificial ageing, the β"-precipitates will be formed, with the percentage of which increasing with natural ageing time, and eventually become the main hardening precipitates. In this work, the effect of Sn on natural ageing and subsequent artificial ageing at 250 ℃ in a Mg-rich Al-Mg-Si alloy was investigated by Vickers microhardness measurements and TEM. The results show that adding a small amount (0.2%, mass fraction) of Sn in the Mg-rich Al-Mg-Si alloy can modify the precipitation pathways upon 250 ℃-ageing: when the alloy is directly artificially aged, the β"-precipitates are dominant, whereas when the alloy is subjected to "natural ageing+artificial ageing" treatment, upon prolonged natural ageing time, the percentage of β"-precipitates would not increase but decrease and that of β'-precipitates would not decrease but increase, but ultimately the β"-precipitates are still dominant over the β'-precipitates. The Sn-induced modification of the precipitation pathways can significantly enhance the age-hardening potential of the alloy upon high-temperature artificial ageing. The addition of Sn increases the effective Si-concentration in the matrix, and consequently changes the precipitation pathways in the Sn-free alloy, which is different from the explanation proposed in literatures.