Update publication information. Mitosis deregulation is a key event during carcinogenesis. Alternative splicing spectrum alteration plays a critical role during mitosis shift. However, the molecules responsible for dysregulated alternative splicing driving carcinogenetic mitosis remain poorly defined. Here, we demonstrate that cancer metastasis-associated antigen 1 (MTA1), a well-known oncogenic chromatin modifier, broadly interacts and co-expresses with RBPs across cancers, contributes to cancerous mitosis-related alternative splicing. Using developed fCLIP-seq technology, we show that MTA1 binds abundant transcripts, preferentially at splicing-responsible motifs, influencing both the abundance and alternative splicing (AS) of target transcripts. MTA1 is cytoplasmic and extrachromosomal at mitosis. Through the RNA-association activity, MTA1 regulates the mRNA level and guides the AS of a serious of mitosis regulators to control the mitosis. MTA1 deletion abrogated the dynamic AS switches of variants for ATRX and MYBL2 at mitotic stage, which are relevant to mitosis and tumorigenesis. MTA1 dysfunction causes a defective mitotic arrest, leading to aberrant chromosome segregation, and resultantly chromosomal instability (CIN) in cancer cells and tissues, which eventually contributes to tumorigenesis. Currently, little is known about the RNA splicing during mitosis, here, we present MTA1 as a pivotal RNA-binding protein, functioning to orchestrate the dynamic splicing of mitosis regulators linked to mitosis control in tumorigenesis.