In Chinese, boundary tones can be categorized into simultaneous addition boundary tone (SiABT) and successive addition boundary tone (SuABT). This study focuses on the use of falling SuABT in child-directed speech for preschoolers' word learning scenarios and examines whether SuABT affects children's decoding of lexical tone. Forty-eight Mandarin-speaking preschoolers participated in an identification experiment. A total of 90 familiar monosyllabic words were designed, comprising two types of word pairs: with or without tonal contrast. The target words were embedded in the two types of boundary tones. The results indicate that complex SuABT does not impede the decoding of tonal categories but rather assists preschoolers in acquiring lexical information. Age demonstrates a correlation with the ability to decode tones under both boundary tones, with a more pronounced correlation observed under the SuABT condition. As age increases, this ability gradually stabilizes after 32 months.