Abstract: In accordance with the disadvantages of traditional triplex single-function reciprocating pump，such as the big fluctuating limit of discharge and pressure and the difficulty to adjust discharge and so on，a kind of triplex single-function reciprocating pump driven by linear motors is put forward.The operating principle of this reciprocating pump is introduced，the motion characteristics and phase is brought forward，which can achieve “constant discharge rate”，the influence of motion phase error on fluctuating limit of discharge is discussed.The research results show，based on the given motion characteristics and phase，the main influencing factors of fluctuating limit of discharge are the length of stroke of pump and accuracy of its controlling system，the higher the accuracy is and the longer the length of stroke is，the lower the fluctuating limit of discharge is.Calculating example shows，without air chamber，triplex single-function reciprocating pump driven by linear motors can achieve the fluctuating limit of discharge of crank-sliding block triplex single-function reciprocating pump with air chamber.The research results provide the theoretical fundamental for developing new style “constant discharge rate” reciprocating pump and designing its controlling system.

Key words: triplex reciprocating pump, liner motors, motion characteristics, motion phase, discharge fluctuating limit