The Arsenic (As) accumulation problem of plants has to be solved when exploring scattered As micro-polluted water as a new irrigation water source for food production. Herein, efficient As immobilization (1 mg-As/L) was realized by simply adding TiOSO4 (TOS) solution in the irrigation water during rice seedling cultivation. The free soluble As in the rice culture was reduced by 99.4%. Most of the As was enriched in hydrous titanium oxide (HTO) formed from in situ TOS hydrolysis (HTO-TOS), which possessed a very high specific surface area (347.6 m2/g) and the most abundant adsorption-active hydroxyl group. Adding TOS (100 mg/L) is tens of times better than adding TiO2 nanoparticles and is even better than commercial HTO at a dosage up to 1000 mg/L in As-immobilization ability. On the one hand, TOS treatment transformed soluble As to insoluble forms by the strong complexation between freshly formed active HTO and As. On the other hand, a barrier of titanium and iron plaque was formed on root surface. Benefiting from these two aspects, the As contents in rice roots and shoots were reduced by 74.6% and 95.6%, respectively. The TOS treatment strategy had no adverse influence on rice growth while promoted the root growth by 39.4% and alleviated the oxidative stress of rice seedlings. Considering its simple application manner and good As control performance, TOS treatment has great potential on in situ remediation of As-contaminated water and soil during cultivating diverse agricultural plants.