Measurement of the local dark matter density plays an important role in both Galactic dynamics and dark matter direct detection experiments. However, the estimated values from previous works are far from agreeing with each other. In this work, we provide a well-defined observed sample with 1427 G- and K-type main-sequence stars from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope spectroscopic survey, taking into account selection effects, volume completeness, and the stellar populations. We apply a vertical Jeans equation method containing a single exponential stellar disc, a razor thin gas disc, and a constant,dark matter density distribution to the sample, and obtain a total surface mass density of 78.7(-4.7)(+3.9) M-circle dot pc(-2) up to 1 kpc and a local dark matter density of 0.018 +/- 0.0054 M-circle dot pc(-3). We find that the sampling density (i.e. number of stars per unit volume) of the spectroscopic data contributes to about two-third of the uncertainty in the estimated values. We discuss the effect of the tilt term in the Jeans equation and find it has little impact on our measurement. Other issues, such as a non-equilibrium component due to perturbations and contamination by the thick-disc population, are also discussed.