The invention discloses a method for calculating an earth capacitance of a DC system based on a first-order circuit full response, and relates to the technical field of DC power system detection, comprising the following steps: step A: establishing an imbalance bridge circuit between the positive and negative poles of a DC bus of a DC system; Step B, closing a switch K1, and obtaining a full response circuit of a capacitor C1; step C, closing a switch K2, repeating the process in step B, and collecting a point at 1S and 2S after the K2 is closed to obtain simultaneous equations; and step D, according to U2 (infinite) measured by twice switch closures, substituting an insulation resistance calculation formula to obtain the values of earth resistances RX and RY and capacitors C1 and C2. Themethod constructs a DC system capacitor and an unbalance bridge model through a first-order circuit full response theory, constructs a RC circuit of a DC system through discharge characteristics of capacitors, and makes the system have a composite impedance; and a first-order circuit full response model of a RC circuit is established according to the twice discharge characteristics of the capacitors, and the earth capacitance of the system is calculated according to the time-capacitance characteristic curve of the circuit.