A virtual image display system, comprising: a handheld electronic device, having a first battery; and a virtual image display, coupled to the handheld electronic device and having a second battery, wherein the handheld electronic device is configured to: calculate a power supply time of the first battery; calculate an expected discharge time of the second battery under a discharge condition; compare the power supply time and the expected discharge time to generate a comparison result; and adjust a supply current provided by the handheld electronic device to the virtual image display according to the comparison result; wherein when the comparison result indicates that the power supply time is less than the expected discharge time, the handheld electronic device is configured to activate a power saving mechanism of the handheld electronic device to reduce a power consumption of the handheld electronic device, wherein the power saving mechanism of the handheld electronic device comprises at least one of reducing a brightness of a screen backlight of the handheld electronic device; reducing an antenna emission performance of the handheld electronic device; and reducing a working clock of the handheld electronic device. a handheld electronic device, having a first battery; and a virtual image display, coupled to the handheld electronic device and having a second battery, wherein the handheld electronic device is configured to: calculate a power supply time of the first battery; calculate an expected discharge time of the second battery under a discharge condition; compare the power supply time and the expected discharge time to generate a comparison result; and adjust a supply current provided by the handheld electronic device to the virtual image display according to the comparison result; wherein when the comparison result indicates that the power supply time is less than the expected discharge time, the handheld electronic device is configured to activate a power saving mechanism of the handheld electronic device to reduce a power consumption of the handheld electronic device, wherein the power saving mechanism of the handheld electronic device comprises at least one of reducing a brightness of a screen backlight of the handheld electronic device; reducing an antenna emission performance of the handheld electronic device; and reducing a working clock of the handheld electronic device. calculate a power supply time of the first battery; calculate an expected discharge time of the second battery under a discharge condition; compare the power supply time and the expected discharge time to generate a comparison result; and adjust a supply current provided by the handheld electronic device to the virtual image display according to the comparison result; wherein when the comparison result indicates that the power supply time is less than the expected discharge time, the handheld electronic device is configured to activate a power saving mechanism of the handheld electronic device to reduce a power consumption of the handheld electronic device, wherein the power saving mechanism of the handheld electronic device comprises at least one of reducing a brightness of a screen backlight of the handheld electronic device; reducing an antenna emission performance of the handheld electronic device; and reducing a working clock of the handheld electronic device.