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Solar cell is currently one of the most promising renewable energy technologies. At present, crystalline silicon (c-Si) solar cell is the most widely used photovoltaic cells and cover 90% market share of the world total PV cell production. Front side Ag paste is a key material in c-Si solar cells because it is the main channel of conducting photocurrent. In general, to gain a higher conversion efficiency for solar cell, the low series resistance and the high short-circuit current is required by optimizing Ag paste and its grid. The rheological behavior is very improtant for Ag paste because it can adjust the screen-printing performace and make the grid has a good morphology to enhance the short circuit current and reduce the series resistance for solar cells. Ethyl cellulose is the most common thickeners, which can be used to adjust the viscosity of pastes. And polyamide wax is an important thixotropic agent that can be used to adjust the rheologic performance of pastess. In this work, The additives ethyl cellulose and polyamide wax of positive Ag paste with different viscosity and rheological behavior was studied to optimize the screen printing performance of the Ag paste, the morphology, densification and resistivity of Ag electrodes. The role of ethyl cellulose and polyamide wax on the viscosity as well as on rheological behavior was different. Ethyl cellulose was mainly used to improve the viscosity of the Ag paste while polyamide wax was used to improve capability of shear-thinning of the Ag paste. Because screen-printing performance is mainly determined by rheological behavior, the ratio of ethyl cellulose over polyamide wax became thus critical. By comparing morphology, densification and resistivity of Ag electrodes, we found the ratio of ethyl cellulose over polyamide wax was 1:5 in our case. Besides, the screen-printing performance of the paste with this ratio is also optimized. The development of new Ag paste and the way of optimizing performance of the Ag paste reported in this paper is very beneficial to further improve the efficiency of current commercial silicon based solar cells.