The invention relates to a lubricating structure of a rotating spline in a gearbox. The lubricating structure comprises a spline shaft, a spline sleeve and a pressing plate, the spline shaft and the spline sleeve are matched through a spline pair, an oil discharging hole is arranged on the pressing plate which is fixed at a free end of the spline shaft and abutted against one end of the spline sleeve, a retaining shoulder is arranged at a spline end, opposite to the free end, of the spline shaft and abutted against the other end of the spline sleeve, a plurality of circumferentially-distributed radial oil collection grooves are arranged between the retaining shoulder and the spline sleeve, an annular inclined-bottom oil cavity is further arranged at the spline end, opposite to the free end, of the spline shaft, one end of the annular inclined-bottom oil cavity is communicated with the circumferentially-distributed radial oil collection grooves while the other end of the same is communicated with a tooth groove of the spline pair, an annular oil collection cavity is arranged at the free end of the spline shaft and jointly formed by surrounding of the spline shaft, the pressing plate and the spline sleeve, and one end of the annular oil collection cavity is communicated with the tooth groove of the spline gear while the other end of the same is communicated with the oil discharging hole of the pressing plate. The lubricating structure is simple in structure, sufficient in spline lubricating and capable of improving reliability and economy of the spline.