The invention discloses a large-measurement-range ammeter. The ammeter comprises a current detecting transformer which is electrically connected with the grounding end of high-voltage electrical equipment. The current detecting transformer is connected with a rectifying module and an ammeter head display module successively. The invention further discloses a current detecting method of the large-measurement-range ammeter. The large-measurement-range ammeter has high precision and the precision in whole process of current measurement is in 5%. Passive and direction measurement is realized and no outer condition is required. Furthermore an energy storage manner is used for supplying motion energy for a counting module. No restriction of a using position exists and use in regions without a power supplying condition is facilitated. Because the large-measurement-range ammeter has no internal resistance, the ammeter realizes no-internal-resistance measurement. No influence on original circuit characteristics exists. Large measurement range in 0-50 A is realized. A mechanical ammeter head (ammeter head display module) realizes measurement range exceeding protection. Furthermore high action speed and in-time protection are realized. Furthermore an operation state of equipment can be visually reflected by means of an indicating lamp.