The direct monitoring of low levels of circulating microRNAs (miRNAs) in undiluted human serums is of significant clinical importance for the diagnosis of different diseases in the early stages; however, this is highly challenged by both the short lengths, low abundance and homogeneity nature of miRNAs and the complicated sample matrix of human serums. In this work, we show the synthesis of a new protein-bridged and size-permeable DNAzyme@PEG hollow nanosphere reactor (p-DPHNR) that enables direct and rapid detection of trace circulating microRNAs in undiluted serums from prostate carcinoma patients. The hydrophilic PEG shell of p-DPHNR efficiently reduces protein adsorption; its size-permeability only allows short miRNAs to enter the core cavity and protects the core DNA signal probes from digestion; and the confinement of the NIR signal amplification DNA probes in the nanosphere core accelerates the reaction kinetics and minimizes the serum background fluorescence. Such p-DPHNR exhibits a 1.8 pM detection limit with a short 20 min assay time for miRNA-141 and can realize clear discrimination of prostate carcinoma patients from healthy donors, thereby manifesting its robust clinical applications for the convenient and early diagnosis of various diseases