Recent progress in the in situ molecular detection of living cells has attracted tremendous research interests due to its great significance in biochemical, physiological, and pathological investigation. Especially for the electrochemical detection of hydrogen peroxide (H2O2) released by living cells, the highly efficient and cost-effective electrocatalysts are highly desirable. In this work, we develop a novel type of microporous Co3O4 hollow nanospheres containing encapsulated Pd nanoparticles (Pd@Co3O4). Owing to the synergy effect between the permeable microporous Co3O4 shell and the ultrafine Pd nanoparticles that encapsulated in it, the resultant Pd@Co3O4 based electrode exhibits excellent electrochemical sensor performance toward H2O2, even when the content of Pd in Pd@Co3O4 hollow nanospheres is as low as 1.14 wt %, which enable it be used for real-time tracking of the secretion of H2O2 in different types of living human cells.