We investigated the low-temperature transport mechanism for poly[2,5-bis(3-alkylthiophen-2- yl) thieno(3,2-b) thiophene] (PBTTT). The temperature-dependent transport behavior was studied by varying the drain-source electric field and gate bias. The results suggest that low-temperature charge transport is dominated by direct tunneling at low electric fields, while field emission is prevailing for high electric fields with high carrier densities. However, the obtained barrier heights are remarkably greater than expected in a conventional field emission. We propose a simplified model of field emission through quasi-one-dimensional path with multiple barriers which shows good agreement with the results more clearly. Field emission across the domain boundaries may assist in overcoming the transport barriers induced by the interchain disorder, which results in the weak temperature dependence of conductivities and nonlinear current-voltage relation at low temperatures.