Abstract: As a research hotspot, the horizontal multilateral well has been an important technology used to exploit marginal reservoirs, heavy oil reservoirs, low permeability reservoirs and naturally fractured reservoirs, and produce residual oil in high water-cut oilfield. This paper first studies the mechanism of percolation characteristics of horizontal multilateral wells in conventional heavy oil reservoir. Then based on the mathematical description of production section(main branch and branch) of horizontal multilateral wells, it develops the mathematical model of productivity evaluation for the symmetrical and unsymmetrical horizontal multilateral wells in terms of the coupling of reservoir and production section, according to the theory of potential superimposition, considering the interference among different branches. The model applied to productivity calculation of horizontal multilateral wells in Bohai conventional heavy oil oilfield and deviation between practical production rates and prediction results was below 20%.