A Mach-Zehnder interferometer system based on weak measurement was set up to determinate the concentration variation of molecule by measuring the phase difference change between the two optical paths. The spectrum of the light was recorded to monitor the concentration of trastuzumab (Herceptin), which is a humanised monoclonal antibody, targeted to human epidermal growth factor receptor 2 (HER2). The trastuzumab targeting to HER2 was real-time detected and continuously monitored, the HER2 numbers of COS7 cells on a coverslip was determined at pico-molar level. Our weak measurement enabled method proposes an alternative approach for the concentration detection of molecules, providing a promising functional tool for the quantification of HER2 in cancer cells, possibly promoting fields such as the diagnosis and treatment of cancer.