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Establishing the Kirsten rat sarcoma <i>(KRAS</i>) mutational status is essential in terms of managing patients with various types of cancer. Allele-specific real-time polymerase chain reaction (AS-PCR) is a widely used method for somatic mutations detection. To improve the limited sensitivity and specificity, several blocking methods have been introduced in AS-PCR to block the amplification of wild-type templates. Herein, we used a novel modified oligonucleotide with internucleotide phosphates reshaped 1,3-dimethyl-2-imino-imidazolidine moieties (phosphoryl guanidine (PG) groups) as primers and blockers in the AS-PCR method. Four common <i>KRAS</i> mutations were chosen as a model to demonstrate the advantages of the PG primers and blockers utilizing a customized PCR protocol. The methods were evaluated on plasmid model systems providing a <i>KRAS</i> mutation detection limit of 20 copies of mutant DNA in a proportion as low as 0.1% of the total DNA, with excellent specificity. PG-modification can serve as the universal additional mismatch-like disturbance to increase the discrimination between wild-type and mutated DNA. Moreover, PG can serve to increase primer specificity by a synergetic effect with additional mismatch and would greatly facilitate medical research.