【解决办法】设置subdomains参数，在WMTS的url地址中加上{s}占位符即可，详细代码参考如下：
var wmtsOptions = {
url: 'http://{s}/iserver/services/map-LL/wmts100',
layer: 'RIVER',
style: 'default',
format: 'image/png',
tileMatrixSetID: 'Custom_RIVER',
maximumLevel: 18,
subdomains: ['localhost:8081', 'localhost:8082', 'localhost:8083'],
tileMatrixLabels: ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12', '13', '14', '15', '16', '17', '18'],
tilingScheme: new Cesium.GeographicTilingScheme({
rectangle: new Cesium.Rectangle(1.7856923984979798, 0.37526560245886974, 2.0031185547384606, 0.46793488206123156)
})
}
var imageryProvider = new Cesium.WebMapTileServiceImageryProvider(wmtsOptions)
var imageryLayer = new Cesium.ImageryLayer(imageryProvider)
viewer.scene.imageryLayers.add(imageryLayer)
