This study examined the semi-spontaneous speech produced by 40 Mandarin-speaking mothers during a storytelling task, focusing on the frequency of sentence-final particles (SFPs) and their impact on speech rate. Results showed that although the overall usage of SFPs was higher in the child-directed speech (CDS) than in adult-directed speech (ADS), this increase was limited to declaratives; in contrast, interrogatives showed the opposite pattern. Further analysis revealed that the higher frequency of SFPs in CDS was driven by a higher proportion of interrogatives. For speech rate, CDS utterances with SFPs were spoken faster than those without, a pattern not observed in ADS. Moreover, mothers lengthened the duration of SFPs in both CDS and ADS, resulting in a slower rate for SFPs compared to the rest utterance parts. These findings enhanced our understanding of caregivers' speech strategies when communicating with children and provided new perspectives in the research of Mandarin SFPs.