Constructing diagnosability model is the prerequisite for diagnosability design, and diagnosability model can analyze the diagnosability of faults in the system. Most of the current research results focus on qualitative research, i.e., the evaluation of diagnosability focuses on the qualitative evaluation of "whether the fault can be diagnosed" without further considering the difficulty of quantifying the fault detection or isolation. To address the above problems, this paper proposes a quantitative diagnosability evaluation model based on earth mover′s distance (EMD), which transforms the quantitative evaluation problem of fault diagnostic difficulty into a distance measurement problem between multiple distributions of test data. First, a qualitative model of multi-signal flow diagram for diagnosability analysis is established to describe the functional composition structure of the system and the fault-test-signal correlation relationship; then, based on the qualitative model, a quantitative diagnosability evaluation model based on EMD is established in combination with the data-driven framework, which can not only qualitatively describe the correlation relationship between fault and test, but also quantitatively measure the difficulty of diagnosing In addition to qualitatively describing the correlation between faults and tests, it can also quantitatively measure the ease of diagnosing faults. Finally, the experimental case of a switching power supply of a certain type of equipment is used to verify the effectiveness of the method in this paper, which provides a feasible idea for diagnosability modeling.