As a new solid state additive manufacturing technology, friction stir additive manufacturing is developed based on friction stir welding. For the re-stirring and re-heating phenomena in friction stir additive manufacturing, both experimental and numerical methods are used for analysis. Monte Carlo method is used to calculate the microstructural evolutions. The precipitate distributions are calculated by the developed precipitate evolution model. The hardness distributions on different additive manufactured layers are then calculated. Experimental data is compared to show the validities of the numerical models. Results indicate that different grain sizes and morphologies can be found due to the existences of restirring and re-heating. The variations of particle numbers and mean radii of precipitates on different layers, caused by different temperature histories, can lead to the different mechanical properties. The mechanism for the generation of different mechanical properties in different layers are explained by numerical simulations in combination with experimental validation.