Sphere-like　mesoporous　CdS　nanocrystals　with　high　crystallinity　and　big　surface　area　were　successfully　fabricated　by　ultrasound　irradiation　at　room　temperature.　The　as-synthesized　CdS　with　and　without　ultrasound　irradiation　was　investigated　by　the　characterizations　of　X-ray　diffraction(XRD),　scanning　electron　microscopy(SEM),　transmission　electron　microscopy(TEM),　and　Brunauer-Emmett-Teller(BET)　surface　areas.　The　photocatalytic　activity　of　the　CdS　was　evaluated　by　photocatalytic　degradation　of　methyl　orange　under　visible　light(lambda＞420　nm)　irradiation.　A　possible　mechanism　for　the　formation　of　the　CdS　nanocrystals　with　mesoporous　structure　under　ultrasound　irradiation　was　proposed.　The　results　show　that　both　the　template　role　of　the　triblock　copolymer　of　P123　and　the　effect　of　ultrasound-induced　aggregation　are　mainly　responsible　for　the　formation　of　mesoporous　structure.　On　the　other　hand,　the　energy　generated　from　acoustic　cavitation　can　effectively　accelerate　the　crystallization　process　of　the　amorphous　CdS.