When thin film resistors are directly written by laser, auxiliary material can cause impurity pollution. Thickness and roughness of the film cannot be reduced easily. To reduce impurity pollution, film thickness, roughness and improve the performance of microcircuit directly written by laser, the research of metallic film resistors directly written by transmissive laser is carried out. The characteristic relation of laser spot energy distribution and film line width is analyzed. The conductive film of line width less than spot dimension can be obtained. The experimental results show that the increase of scanning speed or the decrease of defocusing amount can lead to the decline of overlapping cofficient of spot and line width decline. Meanwhile, excessively high laser power can lead to abliative damage to thin film; excessively low laser power cannot melt the powder on the glass substrate. With different powders in the process, it is known that Zn powder can form a uniform thin film while Al /Zn powder cannot form a uniform thin film.