Abstract: GM tube is a kind of widely used nuclear radiation detector, whose linear range is rather narrow. Major drawbacks were analyzed by adopting dead time models and function fitting. The principle of Time-to-Count method was introduced. The design scheme of an experimental apparatus based on Time-to-Count method was pre-sented. The problems such as pulse-controlled high voltage driving and time interval measuring were discussed. Experimental apparatus was tested in National Defense Science Technology Ionizing Radiation First-order Metrology Station and test data show that the range is extended to 5 orders of magnitude and non-linearity is less than 10% in the whole range.