Tubesheet-flanged joint is usually used in some situations for its detachability, such as some small scale heat exchangers. This type of structure is convenient for maintenance. But leakage will become a major failure mode for flanged joint, especially for heat exchangers with multiple passes. Temperature gradient of different passes may cause non-axisymmetric thermal loading, stress and deformation distribution. In this paper, a detailed three dimensional finite element model of a tubesheet-flanged joint of an oil-water heat exchanger is established with a flexible graphite metallic spiral wound gasket. This kind of gasket has a good compression resilience performance. The compression-unloading curve of gasket under different temperatures are used. The stress and temperature distribution of the joint and gaskets are simulated under preload condition, pressure condition and operation condition. The seating performance are evaluated using gasket compression stress and flange stiffness criteria. The results show that the gasket compression stress distributions under preload and pressure condition are similar. Under operation condition, the gasket compression stress in local area decreases significantly and the gasket compression stress distribution along the circumferential direction becomes non-uniform because of different pass temperature. The Taylor-water method, typically used for flange design, provides a design bolt load for sizing the bolt and flange, does not provide an explicit calculation for the assembly bolt load. At some locations the compression stress is much smaller than minimum seating stress using Taylor-water method to determine bolt preload. However, under preload of half bolt yield strength recommended by ASME PCC-1, all assessment parameters are within the allowable limits.