Stress corrosion cracking (SCC) is considered as the main risk of tubing steels during the exploitation of oil and gas fields, which could result in sudden and catastrophic failures of downhole tubing. Especially in annular downhole environment, P110 tubing steel is prone to sulfide stress corrosion cracking and hydrogen embrittlement (HE) where S^2- could be originated from bio-reduction of SO₄^2- inspired by sulfate-reducing bacteria (SRB). Currently, extensive work have been performed to investigate the influence factors on SCC and mechanism of tubing steels, but limited researches have been conducted on the SCC of P110 tubing steel in annular downhole environment, particularly, on the early detection of SCC. In this work, the SCC behavior of P110 low alloy steel in simulating sulphur-containing annular fluid (SAF) and the effect of S^2- concentration on the initiation and propagation of crack were investigated by slow stress rate test (SSRT), non-destructive electrochemical noise (ECN), SEM and EIS techniques. The results showed that, during the elastic stress stage, the addition of S^2- accelerated the breakdown of passivation film on the surface of P110 steel tensile specimen. There are many short duration current transients caused by metastable pits on ECN curves. The transformation time of metastable to stable pits is shortened significantly by the addition of S^2-, which not only promotes the growth of pits but the initiation of cracks from the stable pits under the action of tensile stress. Compared with the ECN spikes from metastable pits, the spikes associated to the advance of cracks are featured by longer average duration (about 400 s), stronger amplitude (40 μA), and higher charge (about 4000 μC). As a result, the susceptibility of P110 steel to SCC increases with S^2- concentration, and the propagation of SCC is dominated by anodic dissolution characteristic of discontinuous advance.