The perception of lexical tones is a well-known challenge for L2 learners especially in continuous speech where the tonal variations are complicated. To investigate whether the stress, by affecting the acoustic manifestations of tones, has an effect on Mandarin tonal perception for L2 learners, we carried out a perceptual experiment based on the Annotated Speech Corpus of Chinese Discourse with 25 native Spanish-speaking participants. The results indicate that: the perceptual accuracy of stressed tones is significantly higher than that of the unstressed ones in general; T3 is the most difficult one to be perceived among the four Mandarin tones (T1-T4) in both stressed and unstressed syllables, and presumably the Spanish-speaking learners' perceptual order of Mandarin tones is T4-T1-T2-T3; the significant interactive effect found between tone and tonal context in continuous speech may lead to great confusion of tonal perception, especially when T2 and T3 are adjacent with one another.