How is semantic information in the mental lexicon accessed and selected during reading? Readers process information of both the foveal and parafoveal words. Recent eye-tracking studies hint at bi-phasic lexical activation dynamics, demonstrating that semantically related parafoveal previews can either facilitate, or interfere with lexical processing of target words in comparison to unrelated previews, with the size and direction of the effect depending on exposure time to parafoveal previews. However, evidence to date is only correlational, because exposure time was determined by participants' pre-target fixation durations. Here we experimentally controlled parafoveal preview exposure duration using a combination of the gaze-contingent fast-priming and boundary paradigms. We manipulated preview duration and examined the time course of parafoveal semantic activation during the oral reading of Chinese sentences in three experiments. Semantic previews led to faster lexical access of target words than unrelated previews only when the previews were presented briefly (80 ms in Experiments 1 and 3). Longer exposure time (100 ms or 150 ms) eliminated semantic preview effects, and full preview without duration limit resulted in preview cost, i.e., a reversal of preview benefit. Our results indicate that high-level semantic information can be obtained from parafoveal words and the size and direction of the parafoveal semantic effect depends on the level of lexical activation.