The invention discloses a portable nano-machining online measurement device and measurement method. The portable nano-machining online measurement device comprises an optical measuring head and an electric control module. The optical measuring head and the electric control module are fixed through an optical measuring head baseplate. The light emitted by a tunable infrared laser resonator and a voltage stabilized laser diode is connected with an optical fiber transmission module through an optical fiber. The optical fiber transmission module couples the two paths of laser and transmits the laser to an optical fiber collimator. The optical fiber collimator projects the light to the blazed grating. The zero-order diffracted light of the blazed grating is projected to a reference arm reflector and returns according to the same way, and the first-order diffracted light is projected to the surface of a measured object through a collimating lens and returns according to the same way. The two paths of light interferes and returns to the optical fiber collimator, and the optical fiber transmission module detection an interference signal. The electric control module comprises a power supply unit and a signal acquisition and processing and system closed-loop control unit. The functions that optical measuring head power supply can be realized, the interference signal is transmitted to the computer to perform data processing and the system signal deviation is transmitted to the piezoelectric ceramic power supply unit to stabilize the working point of the system can be realized.