Traditional therapeutic and diagnostic tools exhibit serious side effects, poor selectivity, and sensitivity. Herein, a multifunctional CuPc@HG@BN theranostic platform composed of hexagonal boron nitride nanosheets (h‐BNNS), conjugated DNA oligonucleotide, and copper(II) phthalocyanine (CuPc) was developed in which the CuPc molecule played double key roles in photodynamic therapy (PDT) as well as in situ monitoring and imaging of miR‐21 by surface‐enhanced Raman spectroscopy (SERS). Owing to the designed circle amplification of miRNA and high SERS effects of CuPc on h‐BNNS, miR‐21 responsive concentration was achieved as low as 0.7 fm in live cells. Both in vitro and in vivo data demonstrated that the integrated nanoplatform showed remarkable enhancement in PDT efficiency with minimized damage to the normal tissues. The developed probe was also successfully utilized for early monitoring and guiding the early therapy, realizing tumor elimination. A theranostic nanoplatform composed of h‐BN nanosheets, conjugated DNA oligonucleotide, and copper(II) phthalocyanine was constructed for SERS in situ monitoring of miR‐21 and photodynamic therapy. Real‐time monitoring of miR‐21 with this ultrasensitive, highly selective, and accurate theranostic nanoplatform has been successfully utilized for guiding early theranostics to improve breast cancer therapy efficiency.