The invention discloses a portable water purifier and water purifying, back washing and packing replacing methods and belongs to drinking water treatment. The portable water purifier comprises a water drawing part and a filtering part; the water drawing part comprises a feedwater barrel, and an inner thread is arranged in the lower end of the feedwater barrel; the filtering part comprises a packing barrel opened in two ends, each of two ends of the packing barrel is provided with a double-end stud, and one of the double-end studs is matched with the inner thread of the feedwater barrel while the other one is connected with a tail hood with a water outlet. By the arrangement, structure of the packing barrel is purified, packing can be backwashed on the basis of meeting requirements of water purification, change frequency of the packing is lowered, service time of the packing is prolonged, utilization rate of the packing is increased, and replacement of the packing is much easier; with the method, sewage can be purified to drinking water standards by the aid of the water purifier; washing of the packing of the water purifier can be realized through the back washing method, and service time of the packing is prolonged; the packing can be easily and rapidly replaced with the packing replacing method.