Starting soon after discovery, we carried out a programme of continuous observations for the active comet C/2017 K2 (PANSTARRS) using the Lijiang 2.4 m telescope from 2017 September 8 to 2018 June 18 with a Johnson R filter. Three fixed linear apertures of ra1 = 6.4 x 104 km, ra2 = 7.7 x 104 km and ra3 = 9.6 x 104 km were chosen to measure the brightness of C/2017 K2 respectively, and the instrumental magnitudes were then converted into the PS1 r system. During the period of observation, the cross-section area increased at rates of 22.0, 28.5 and 44.1 km2 d(-1) relative to the different linear photometry apertures, and the corresponding mass-loss rates were 63.8 kg s(-1), 82.6 kg s(-1) and 127.5 kg s(-1), assuming that the dust-particle size is (a) over bar = 0.5 mm and the density is. = 500 kgm(3). The size of the C/2017 K2 nucleus is estimated as 1.2 less than or similar to rn less than or similar to 2.5 km for CO sublimation and 3.1 less than or similar to rn less than or similar to 183.9 km for CO2 with an aperture of 9.6 x 10(4) km; meanwhile the grain size of K2 is ac less than or similar to 9 mm for CO and a(c) less than or similar to 0.8 mm for CO2. The long-term monitoring data provided in this paper will be very valuable and helpful for studying the activity mechanism of C/2017 K2.