This study investigates the phage resistance mechanism of CRISPR1 in *Streptococcus thermophilus*. CRISPR1 is distinct from known phage defense systems and effectively protects *S. thermophilus* against two main groups of phages. Analysis of 30 bacteriophage-insensitive mutants (BIMs) reveals that the addition of a new spacer in CRISPR1 is the most frequent outcome of a phage challenge, and iterative addition of spacers increases overall phage resistance. The newly added spacers are 29 to 35 nucleotides long, with 30 being the most common size. Comparative analysis of 39 newly acquired spacers with wild-type phage genomes shows that the spacer must be identical to a region (named proto-spacer) in the phage genome to confer resistance. A CRISPR1-specific sequence (NNAGAAW) located downstream of the proto-spacer is crucial for phage resistance. Additionally, virulent phages rapidly evolve through single nucleotide mutations and deletions in response to CRISPR1. The study confirms that CRISPR1 is a novel and effective phage resistance mechanism in *S. thermophilus*, with potential applications in fermentation and biotechnological processes.This study investigates the phage resistance mechanism of CRISPR1 in *Streptococcus thermophilus*. CRISPR1 is distinct from known phage defense systems and effectively protects *S. thermophilus* against two main groups of phages. Analysis of 30 bacteriophage-insensitive mutants (BIMs) reveals that the addition of a new spacer in CRISPR1 is the most frequent outcome of a phage challenge, and iterative addition of spacers increases overall phage resistance. The newly added spacers are 29 to 35 nucleotides long, with 30 being the most common size. Comparative analysis of 39 newly acquired spacers with wild-type phage genomes shows that the spacer must be identical to a region (named proto-spacer) in the phage genome to confer resistance. A CRISPR1-specific sequence (NNAGAAW) located downstream of the proto-spacer is crucial for phage resistance. Additionally, virulent phages rapidly evolve through single nucleotide mutations and deletions in response to CRISPR1. The study confirms that CRISPR1 is a novel and effective phage resistance mechanism in *S. thermophilus*, with potential applications in fermentation and biotechnological processes.