Abstract: In order to explore the distributions and influencing factors of total dissolved inorganic arsenic(TDIAs, TDIAs=As⁵⁺+As³⁺)in the Pearl River Estuary(PRE)and the northern South China Sea (NSCS), two cruises were carried out in October 2014 and June 2015. The concentrations of TDIAs were measured by Hydride Generation-Atomic Fluorescence Spectrometry (HG-AFS) method. There was no significant difference between the average concentrations of TDIAs of adjacent area of the PRE in summer and autumn (t-test, with significant of level 95%). TDIAs showed non-conservative behavior during the initial mixing of fresh and saline waters in the PRE in both summer and autumn, with scavenging efficiencies of nearly 30%. The transport flux of TDIAs from the PRE to the SCS was (4.0~4.6)×10⁵ kg·a^-1. The concentration of TDIAs was higher in the adjacent area of the PRE. The concentration of TDIAs increased at the depth of 100 m in the northern slope of the SCS due to the incursion of the Kuroshio Subsurface Water. There was no significant difference about the average concentrations of TDIAs in the similar area of NSCS between two seasons. TDIAs showed conservative behavior in the NSCS in summer. The distributions of TDIAs in the NSCS is mainly controlled by the mixing of different water masses. Compared with other areas in the world, the concentration of TDIAs remained at natural levels in the PRE and NSCS.