This paper investigates the influence of three factors on pronunciation accuracy of Chinese adult foreign language learners. Ten target sounds including phonemes and syllables are included in the pre-test, an analysis of which shows that the mispronunciation of the randomly chosen target sounds mainly results from L1 negative transfer. It is observed in intervention that some mispronounced target sounds are more difficult to be corrected than the others. However, the hierarchy of difficulty for pronunciation acquisition cannot be constructed without considering the impact of task variables, for even the same subject's performance might vary in post-test including vocabulary reading, sentence reading and spontaneous speech, in which tasks individual aptitude in perception, mimicry and monitoring also has a role to play in the improvement of pronunciation accuracy.